Our son has always had a good bike. My DH absolutely doesn't trust the cheapy bikes because of brakes not working as well and such.
He started on a little Trek with training wheels and once he learned the bike was still good for him to use without the training wheels. Then we got him a specialized, and now he has a new Trek that should last him a while.
Kids are hard on bikes but if you get them a good one than they components and are all adjustable, you can't upgrade them, and replace things on them, where as the cheapy bikes aren't really worth fixing up if something happens to them.

Just my opinion but that is how we have done things with ours. You can sometimes find them used and still in really good shape at the bike shops.
We have bought them new, and the kids bikes aren't nearly as expensive even new. Of course I guess you can get them that way, but depends on how much you have to spend.