BTDT.
Avoid the seven speeds. Any that I have seen, have a "twist grip" shifter. Apparently no designer in Japan ever asked a kid for their opinion, because those things are unusable junk.
Overall Overview: What you pay now, is NOTHING compared to carbon 29ers and snowboards, and probably also wrecked cars and college.
Spend just a bit to get something nice, and your kid will not hate riding.