Some kids are just crafty. They're the ones who make mudpies when they're 2 and by 3 they've nicked Grandpa's epoxy to stick their blocks together. Seriously, if your kid is trying to make things, you want to channel that somewhere safe before your whole house is covered in crayon and play-dough!
Conversely, some kids are NOT crafty. They're the ones bored by fingerpaint, who might have thrown food but never really played with it, who whine that craft projects are stupid. And for them it's true. They need different projects. Maybe they'll develop more patience for crafts later, but certainly not if you force them and they wind up hating it.
One thing craft projects are great for at any age, however, is teaching how to follow STEPS. First you talk about the project. Then you lay out your materials to make sure you have everything. Then you do the craft one step at a time. Afterwards, you look at it to see how well you did. Finally you clean up the mess. Kids who start learning this as toddlers are kids who read the recipe as tweens and therefore do not wind up with half a cookie batch stirred up before they realize there are no eggs.
For preschoolers, crafts need to be really simple. I grew up helping my parents and grandparents with projects -- making quilts, growing a garden, building birdhouses, etc. But one of my favorites from a very early age was making leaf windows from autumn leaves, shaved crayons, and waxed paper. In addition to the fun of hiking through the neighborhood collecting leaves, I learned the names of the trees, the colors and shapes of the leaves, that you had to make fine shavings or they wouldn't melt right, that if you hacked at a crayon it would break, and so forth.
It's about progression as much as expression. First collect the leaves. Then put them on waxed paper. Then shave crayons over them. Finally put the top page on, cover with cloth, and watch while Grandma irons it. Tape to windows and admire. I can remember that when I was very tiny, the only things I did were pick up leaves and put them on the bottom page. As I got older, though, I started tearing off the waxed paper, which takes a bit of skill that is useful in the kitchen. I learned to shave the crayons. And toward the end, I learned to use the iron, which is good practice for ironing other things such as pattern tissue and garments. The craft was fun because it was pretty and we got to spend time as a family, but it was also useful because it taught a lot of things along the way.
When you're thinking about whether to do crafts with wee ones, your own or someone else's, ask: Is this kid showing interest in making things? If so, what kinds of things do they like and what haven't they tried yet? What are your favorite memories of making stuff as a kid? What materials are easily found at this time of year? What does the craft teach along the way? Does it have a progression of skills, either adding more complex features over the years or allowing the child to take on more of the tasks with maturity?
Just yesterday we went to a botanical garden and I naturally went into Nature Show mode, because it's what I do walking in a garden or forest or zoo with people, because I was raised by two teachers who just habitually point at things and name them. It's fun to identify things, or guess, and check the tags to see if you're right. We pointed out hummingbirds and a nest full of some other sort of quite noisy hatchlings and squirrels scampering around a tree. That stretches from early leaf walks through current ones to being able to figure out what an alien tree is for when I'm standing on a planet far far away trying to figure out what affects the plot. Fruit tree, shade tree, tree that drops venomous symbiotic insects, main character shaped like tree. These things matter, and they have their roots in growing up with a very experimental and experiential approach to life.