I think girls would probably be pretty easy, maybe it's just b/c I am a girl, but I would do things other mentioned like hair accessories, chapsticks (bonnie bell type) and lipglosses (BB also makes those and they appear more mature, even though they're not;), socks, etc.
For boys, maybe sports watches (you can find fairly inexpensive ones at Wal Mart, etc.), also probably socks, deodorant, small leggo sets, handheld games or card games (they make reasonably priced ones in the card game section at places like Target, etc.), maybe trading cards like baseball or something?, bubble gum
I don't know how many you plan on making, but those are my suggestions! Good luck and have fun!!!