Depends on your budget and skill level.
Hamiltonkiler's suggestion may be the best for a 5~8 y.o.. Hit a few gunshows and pawn shops, a decent youth size entry level rifle should turn up for around a C note. A new Keystone Chipmunk is about $250. Otherwise, I'd go to Numrich and see what they have. Then to Boyd's. An auction site [Gunbroker or fleabay, etc] is also a possibility. Also, check out the Rimfire Central forum.
Good Luck on your project. When my G.S. is ready to learn firearms, I'll pull my old J.C. Higgins single shot that I learned on and taught his dad with out of the safe. It is a noble quest and you are obviously putting some effort into it.