Since Charles was young, I'd say 18 months old or so, I've allowed him to help me in the kitchen. I let him do easy things like pour in thing's I measured and stir things in a bowl. As he grew older, I allowed him to do more, like wash vegetables and peel potatoes. When Tommy got old enough - he began to help out in the kitchen as well. Tommy is wicked fast with a veggie peeler.
I've used kitchen things to teach skills like fractions. We need 2 teaspoons of vanilla. How many 1/2 teaspoons do we need to get 2 whole teaspoons. Making things like gingerbread has been a good science lesson. There is something about the baking soda foaming up the acid that just makes little boys giggle.
Charles is now accomplished in the kitchen. He can make omletes and can follow recipes from a cook book.
Joey is always in the kitchen with me. As soon as he thinks I am headed that way, he comes running, dragging the green chair behind him.
What about the mess you say? There is none. If you teach them to do it right, the mess is minimal and then you teach them another important life skill - ClEANING UP!!!! Cleaning up is an important a skill to learn as cooking. Trust me, nothing shows how important it is to practice good clean up skills like scrubbing baked on egg out of an omlete pan.