C.V.
Kids that old will probably bring stuff with them to do. My 3rd grader likes to play video games (probably most kids will have a hand-held game by that age), read, draw pictures and watch movies. If they will have access to a backyard, that is even better (if you can trust them not to climb the fence).
I wouldn't plan too much for them to do before you get to know them better. 3rd graders have pretty definite likes and dislikes. I can assume that 6th graders are even more so. You might want to find out what they like to snack on before hand & stock up. They always seem to be hungry.