This time of year you can buy squirt guns fairly cheaply (especially at the dollar stores). Squirt gun painting is a fun way to occupy the children for a while.
Squirt guns (one for each color paint)
Paint (you probably need something washable that you can water down)
Paper (water color works best but posterboard might be another good choice)
Tack the paper to a tree with thumb tacks (make sure you don't drop any and that the children keep shoes on).
Put the paint filled squirt guns in a bucket for them to choose