For all my kids at that age I turned to the used market. If the bike went through the first kid and didn't suit the next I could then sell it for nearly the same money if not close enough. Otherwise they grow out of the bike so fast you hardly get your money's worth! Also at that age for us were 20" bikes, we're all on the smaller side. At that time, used, I found a Gary Fisher Pre-Caliber for $150 at a local bike shop (extremely light) and a Diamondback mountain bike for about $100 off Craigslist.
That said I did buy a Raleigh Scout 24" brand new a few years back and it's gone through all three kids.