My thirteen month old, Bobby, plays on the floor. At this point, he enjoys a whisk and metal bowl. He doesn't try to help beyond that. I am convinced, though, that as he sees his older brother and me working, he will want to help with various tasks that seem interesting.
My three year old, Daniel, loves measuring flour. Flour is measured by a big cup, and usually it takes several cups to get the right amount. We count aloud until it is time to stop. Sometimes, if I use only a cup or so of flour, I use a smaller measuring cup so that we can still count. He loves the texture of the flour and uses his hands to fill the measuring cup. At first, it was real messy, but he's getting the hang of it. If he makes a floury mess on the floor, I hold off sweeping it up and let both kids play with it until I am done cooking, because flour is fun to play with.
Mixing and Mashing
Daniel likes to stir. This generally does not slow me down. Neither does it slow me down if he grabs a fork and mashes potatoes with me. With most other tasks, his help does slow me down, but we have fun, so it is no trouble.
This one is fun. I like kneading dough, but it took me a while to catch on. I've heard that men are often better kneaders, as they have stronger arms and hands and can get it done faster. Teaching him to knead is an investment in my relationship with my future daughter-in-law as well as wicked cute.
I let my son count out the potatoes from the bag to the pan. He can count to eight, and occasionally slips in a nine, so this task is given over to him independently. After I wash them, he passes me one potato at a time to chop.
My son is three and his "help" does not, at this stage, contribute anything to the cooking but a good time. However, the benefits later on will be immeasurable. The fact that cooking is fun in our home means he will be my kitchen helper for years to come. It will also demonstrate to younger siblings that this is fun, and worth doing, so I will have a bunch of kitchen helpers.
Passing down the love of cooking teaches children a life lesson. Convenience foods are for people who can't cook, or don't find it enjoyable enough to devote their time to it.
Counting in the kitchen is just the beginning. Lessons in arithmatic, fractions, and algebra will find their way into the kitchen. Physics and chemistry already have.
This wasn't something I intentionally sought after though, having baby kitchen helpers. But because I love it, they love to be with me while I'm cooking. This, I think, is the key to teaching children anything, loving what you do and working hard at it yourself.