I've watched things over the last several decades go from almost nobody knowing anyone with dietary restrictions, except for a few old people whose bodies were breaking down, to almost everybody having at least one thing they can't eat. Can't have dairy, or sugar, or gluten, or high-fructose corn syrup or whatever else. Consider how common food additives have become. High-fructose corn syrup is in about 95% of food products. Imagine being allergic to the damn stuff but not knowing WHY 95% of food tasted disgusting and/or made you sick. Artificial sweeteners which are sweet to some people are inedibly bitter to others. Preservatives, colors, etc. that are negligible to some are vile to others -- or cause indigestion, or behavioral problems, or some other hassle.
So there you are, already frustrated and miserable because almost nothing is safe to eat. (Forget tasty or interesting. Those are things other people talk about that make no sense to most people with food sensitivity.) And all your asshole relatives do is try to force you to eat food that makes you sick. Terrific.
Sometimes people have brain weasels that make them think food is bad when it's fine. But quite often, they are alerting to something that just isn't obvious to other people, but if they eat that food they will throw up. Or get the runs. Or not be able to sleep for three days. Or some other thing that makes them miserable. Or all of the above. :P
Some things that can help ...
* Pay attention to your body (or your child's body if they're too young to track it themselves yet). Most people with food sensitivities have loud and clear signals, unless somebody messes them up. It helps to distinguish between physical and psychological aversions, since they have different solutions or ameliorations. You might talk yourself out of a philosophical aversion to trying unfamiliar things, but you're not going to talk your way out of lactose intolerance.
* If something makes you throw up, or causes other problems, don't eat more of it. With foods that mix ingredients it can take a long time to pin down the exact ingredient causing trouble. Plenty of folks wind up with broad categories like "artificial flavors" or "highly processed foods" that are more likely than not to cause a bad reaction.
* Figure out what things you can eat safely. Try to find ones from different food groups if possible.
* Look for things that are related to things you can eat safely. Frex, if you can eat red tomatoes, try yellow ones. Sometimes a difference in color signals different nutrients. If you can eat plums, try nectarines.
* Look for things that are related to things you CAN'T eat safely. You should probably avoid those too, unless there's a specific reason they might work better. Frex, if it's acidic food that makes you sick, red tomatoes have more acid than yellow ones. So the red might be inedible but the yellow might be okay. But if you can't digest beef, then pork probably isn't any better; they're both fatty meats.
* If a recipe fails out because of only one or two ingredients, you may be able to save it. Can't eat dairy? Try margarine or almond milk. I swapped out the butter for margarine and the milk for halvah in creamed yams and got a wonderful new recipe. Don't like peppers? They can be left out of most casseroles or other mixed dishes. Can't digest meat? Try swapping in mushrooms, tofu, or vegetarian meat substitute.
* Consider trying a whole different cuisine. Sometimes people just wind up in a culture whose whole food system disagrees with them. Like if you can't digest rice you are screwed with Chinese or Japanese cuisine, but hey, German uses potatoes for the starch base instead. If you can't digest meat, then check out the huge array of vegetarian main dishes from India.
* Know why you're having problems. Autistic spectrum? Control is probably an issue. Try making your own food. Even if you can only learn a few simple things, it gives you a little more control. Autism and allergies mean you probably need to try new foods gradually. Change only one variable at a time. But maybe you like the idea of having a wider diet, just have a damn hard time finding anything that tastes good. Get together with some friends at a "Taste Of (City)" Festival and buy a whole bunch of new things to share and try. With a little luck you may find something new that you like. Booths field really uncommon things and you can find things you wouldn't otherwise encounter. Cross-contamination? Try divided plates like people use for picnics. They keep each food distinct. Little sauce cups work too. Sensory processing disorder? Figure out specific textures or flavors that cause upset. If you like crunchy but not mushy, try raw instead of cooked carrots (or vice versa). Cooking methods like blanching can reduce bitterness.
* Seriously keep an eye on modified food. Processing. GMOs. Some hybrids. Artificial colors, flavors, sweeteners, preservatives, etc. You may be allergic to one but not another, or to a whole category. But if you can't tell the blasted things apart because of lax labeling, you're stuck skipping the whole category or else playing Porcelain Altar Roulette. Organic food is more expensive but often safer. A drawback is that processed "health food" is frequently just not yummy. Edible is better than inedible but not by much. So single ingredients that you fix up yourself may be better than premade stuff.
* Somewhat related to this: vitamins. If you're not getting a balanced diet, but you can tolerate vitamins, they can prevent some of the serious nutrient deficiency diseases. They're not as good as real food, but they are a great safety catch. Trouble is a lot of them use -- what else? -- artificial colors, flavors, binders, etc. There are organic ones that may be safer.
* Is it easier or more pleasant for you to eat food prepared a certain way? Just do that. Cut the crusts off the sandwich. Turn the carrots into coins or cubes or whatever floats your boat. There are whole industries devoted to making food more appealing. Do whatever works for you.
* Another thing to try for digestive problems (but usually doesn't work for psychological ones) is pre-digested food. That means fermented or macrobiotic anything. Yogurt, cheese, sauerkraut, certain types of pickles, sourdough bread, etc. Also any kind of marinade, fruit topping, or other enzyme carrier that quickly breaks down whatever it touches. We discovered this with the Amazing Mango Ham, for example. If your body is just not very good at breaking down food, then giving it food already broken down is a huge help.
* For psychological problems it's almost the opposite. Don't fixate on the contents but on the presentation. Sometimes measuring food helps a lot just so you know when you've eaten enough. I can almost never finish an ordinary serving so I experimented until I found some dishes that are me-sized servings at home, and memorized the amount. This means packaged things like pudding cups or fruitbars can be a godsend. For some people, certain shapes are easier to handle. If you need to eat all finger foods, then go for it. If chefs can spend 20 minutes plating haute cuisine then you can cut your radishes into flowers if that makes them acceptable.
* And while we're on the topic of serving size, the American meal system is NOT biological optimum. Look at other cultures' meal patterns and try them, or make up your own. Second breakfast is great if you get hungry between breakfast and lunch. The Mexican system does well putting the main meal in the afternoon instead of at night. If it's too hard to eat big meals, then don't. Eat small frequent meals. Conversely, some people do better gorging 3/4 of the day's calories in one go and then just snacking the rest of the day. But you get two basic patterns from all this: browsers/grazers and gorgers. They do not do well on the wrong pattern of food intake. Their dietary planning needs to be different. As a browser, I know that I need highly nutritious snack foods: fruit, fruit bars, a loaf of pretty good bread, etc. A gorger can eat any junk they want for snacks but really needs a well-balanced main meal.
* Unless your brain lies to you, cravings are your friend. They tell you that you're not getting something you need and should go eat it. If you have brain weasels, you will need to find other ways of figuring out what are valid signals from your body and what are weasel whispers to be ignored. The trick with cravings is to remember they're not about foods but about nutrients. Try to sort out what's high in the food you're craving, and whether that's the best source or you could find a denser one.
* If eating is hard due to negative associations from people and/or your body, there are at least two ways to get around it that help some (mostly different groups of) people:
1) Ignore the food. Eat while you are doing something else and concentrate on watching TV or happy conversations, etc. Sometimes you can consume an amazing amount while not paying attention and just eating on automatic. Most bodies have kind of an auto-graze-mode, and that's why weight-loss diets tell you to pay attention only to eating, not eat while you are doing something else. When the goal is to increase food intake, just reverse the advice for manipulating the same trait.
2) Anchor with positive associations. Negative associations can be erased with time and care. This can be as simple as making mealtime conversations pleasant instead of stressful. Anything you enjoy, or any happy memory, can be used to whack down the level of stress, though. Find one that works for you. If eating is hard and you're doing it, you deserve praise and reward!
* Buffet restaurants help if you are concerned about size/diversity of portions. You can try as many different things as you want, and only take a tiny dab of each. A drawback of this approach is that you may not know exactly what is in the food: a problem for many folks with allergies. But if your problems are more in your head, or if you can tell with a sniff or a lick what's safe and what isn't, buffets give you lots of options. Quick-cook buffets are great because they let you choose which raw ingredients you want grilled or stir-fried or whatever. You won't have to pick out onions or peppers or other gross stuff!
* If you're trying to expand the range of edible foods, push gently. Pushing hard tends to cause a rebound that not only increases stress but shrinks the range of acceptable foods. That is the opposite of helpful. Do what you can. Sometimes that is just looking at pictures or recipes of different foods to see if anything seems remotely appealing.
* People who pick on your about your food are not helping and not your friends. Avoid them if you can. Otherwise try to ignore them. Their opinions matter less than finding food you can eat without suffering gastric or emotional distress which exceeds the nutrient value gained.