Here are some of our favorites specifically for kids:
(Ages 4-8)
Sesame Street Silly Snacks Cooking with Kids (out of print but plenty available on eBay).
Anything in the
America's Test Kitchen for Young Chefs series
With a little help, your 8yo should also be able to handle these:
Rachel Ray's Look + Cook (lots of photos, steps broken down, not many crazy ingredients, ~30-45 min)
The Best Simple Recipes by America's Test Kitchen (for an ATK cookbook, they really do keep it simple!)
Teach them good knife techniques and to clean up as you go. Put them 'in charge' of dinner one night a week (even if you're the sous chef!)....and enjoy this journey of them learning a lifelong skill and pleasure.