The 4 year old - how about G.I. Joes? My kids loved them - their friends also loved them as they seemed to "follow" them home. You could also get a sm box (shoe box size) and put camafloge tape on it for Joe to "sleep" in.
The 2 year old is the tough one 'cause I'm sure he tries to keep up with older brother. Maybe some bigger cars or trucks.
You could also go with puzzles, board games, card games, coloring books...
Boy am I not awake I read sister the first time not sitter!
A gift cert. to a nice resturant would be nice for the mom and dad. Maybe offer to watch her kids for a few hours.