A.B.
Bake square-shaped flag cookies and tint store-bought frosting. Give each kid his/her OWN plastic cups of red and blue frosting and little spreaders because they'll be licking their spreaders. (Especially the toddlers--with all preschoolers you may be able to let them share.) You can also provide star-shaped sprinkles. Then throw them all in the kiddie pool afterward because they'll be a mess. It does require set-up, but you won't have to actually help them decorate. (provide bibs for the littlest ones)
Easy-to-peel stickers on some type of pre-cut paper is also a good craft for little bitty ones who can't do too much yet, and older kids love stickers too. I bought some sticker Chinese New Year sheets from Oriental Trading for my daughter's class, but they were not too easy to peel and they had to be helped, so I will go elsewhere next time.