Non-toxic modeling clay invites the movement of the hands and fingers when the child is able to roll, shape, cut out and shape the material to their own liking. Some accessories like a little rolling pin and some plastic cookie cutters extend this fun even more.
If your child hesitates, try other materials, as it may just not like the smell or texture of the product. The softer the clay, the easier it is to work with small hands. Edible dough is of course the absolute pleasure. When you bake, give your child some dough so they can knead their "own cake" and bake it with theirs. (If you rarely bake, then get some finished dough from the fridge.)
A mud cake kitchen or a sand castle construction site in the garden also promote dexterity. If your child builds a tower and puts a feather on top as a tower flag, then it does not notice that it trains its fine motor skills - but you notice it!