I know a 7yr old and a 12 yr old with a gameboy advance each (one of those flip top ones). My 8yr old nephew has a nintendo ds (newer style handheld with a pen). They would play with them ALL the time if the could get away with it! I bought my ds1 (5yrs) an older style gameboy advance from Ebay and he never plays it because it has no backlight and he can't see the screen - he likes our PS2 much more.
Nothing wrong with buying things from ebay - just make sure that they have a decent score after their username, and pay with paypal, then you've got backup to help complain if it does go wrong!
Kids go in phases, with the newest thing out being the most fashionable must have. Whatever you get, its going to be unfashionable and superseded sooner or later!
PS2's are pretty good due to the fact they'll play regular cd's and DVD's too, so its a three in one item! Very handy for space saving! HTH