I would definitely re-theme to a (elementary) schoolyard theme.
Locker (Cellar), Playground (Village), Bully (Thief), Cheater (Spy), Teachers (Harem), Principal (Nobles), Lunch Money (Cache), Secretary (Bureaucrat), Library (Library), etc.
Also, I would not include Curses at all.
Probably no heavy trashing or difficult to calculate alt VP, either. Focus on one of the pillars of Dominion (Thief) and the basic game structure; no Chapel, Gardens, or Witch or similar cards.
No Throne Room because of the "must play" -- or a reprinted version with may play (retheme: "Principal's Office"). No Moat (waiting for people to reveal when playing attacks and reactions in general, eh, not so sure that is a great idea). No Chancellor or Adventurer or Feast. Probably the rest of the base set is okay?
Harem and Nobles (rethemed as Teachers and Principal) would be my only alt VP. Cache for alt money (rethemed as Lunch Money). Probably include Warehouse for more sifting, and Treasure Map for the one-shot. Wishing Well is a great one. Maybe Coppersmith? Retheming ideas here?
If I thought about it more I would try to include more interesting combos/interactions, but stay away from cards that promote mega-turns and heavy engines. Some kids will get it, many won't (there is a conceptual barrier here I don't think a lot of young kids will get), which leads to less fun. Games need to be more lucky for kids of various skills to have fun together -- the opposite of adults.
I like Davio's suggestions, too.