I used to do meatballs as a kid, and my bro and sis loved crumbing stuff - I couldn't stand the egg.
Mum let us do packet cake mixes by ourselves - great photo of my sis sitting in the sink with a spoon and ice cream container and chocolate batter all over her. She would have been not even 2 - so cute!

When I was nannying, I would let kids dictate the amounts of certain things so they felt in control, then taste it to see what they thought, then I would suggest things to adjust it accordingly. We got some wacky shaped stuff, but it always tasted good. I sort of discovered with cake (and I have good kitchen sense, I know not everyone does) that as long as it's pretty much the right consistency and the batter doesn't taste too floury/fizzy/salty then it would turn out as cake. Great way for kids to achieve confidence and learn about tasting their food (do I sound like someone off MasterChef yet?)