I spend a lot of time with my cousin's kids and they love getting into the kitchen.
I've found they often lose interest if things are too complicated, so we keep things pretty simple. One of them does Garden to Table at school so wanted me to cook her curry for dinner recently, a surprising departure from her usual home-made fries and chicken.
The other one loves baking and we often make variations of these cookies, changing the chocolate flavour, especially. Sometimes she'll sit and watch the oven timer tick down, other times she'll give up part way through and forget about them until they're ready.
Smoothies are always a good one for kids to tackle, too. You can mix up the colours, depending on the fruit, and they're a quick and healthy snack.
Pancakes were my ultimate food as a kid - these will be well enjoyed by adults and kids alike.