Make sit-upons! Use old jeans and newspapers. )Each girl can bring her own pair of old jeans that they don't mind cutting.)
Cut the legs off below the pockets (like a short pair of shorts). Then, either sew or use basting tape to close the leg holes. Put the newspaper inside (enough to make it comfortable to sit upon) the shorts and (again) either sew or use basting tape to close the top of the shorts together.
Have the girls paint the shorts (we used tempra paint when I was growing up - but fabric paint in tubes works, too) however they want. Then, hang them up to dry (in case the girls paint both sides of the shorts!!)
Use some sort of ribbon or decorative yarn in the belt loops and whalah - situpons! (I actually still have mine and I was probably about 7 when I made it - and I have my daughter's, too - she was about 7)
Good luck & have fun!!!