Star, this is a super-easy muffin recipe I've been making with my oldest daughter since she was 4.
1/2 cup oil (or applesauce)
1 cup shredded carrots, OR two medium mashed bananas
1 tsp baking soda
1/4 tsp salt
1/2 cup milk
1 cup sugar
2 eggs, beaten
1 1/2 cups sifted all-purpose flour
1 tsp baking powder
1 tsp cinnamon
Mix sugar & oil. Add beaten eggs. Stir in shredded carrots (or bananas).
Sift flour, baking powder, baking soda, salt, and cinammon. Add small amounts to sugar mixture alternating with milk.
Bake approx. 20 minutes in preheated 375 oven, in greased or nonstick muffin tins (12 regular-size or 6 jumbo).
If you're making banana muffins, you can toss in some chocolate chips, or add sprinkles.
The best thing about this recipe is that the ingredients are all common things that are always on hand in the cupboards, so if you've got a bored little one, you can suggest making these on the spur of the moment. A kid can also easily mix the wet ingredients and mash the bananas, and pour the milk in while you're mixing (since you shouldn't over-stir muffin batter, I do that part myself).
My oldest just loves making these with me. Her sister is almost old enough to help, too.
Another thing they both like to help with is pancake-making. Their dad is usually on pancake duty here, and he lets the girls break the eggs in the bowl and do the mixing.
Baking is easiest to do with kids, I find, because you don't have to worry as much about bacteria (like you do when handling raw meat). Veggie duty, like Hep suggested, is a good idea too.
That book lildago suggested sounds great, too.