Between clearly kids bikes that cost about $100 from Wal*Mart and grownup bikes, there is a funny zone. Basically, until you get to 5"4", you can't really get on a mainstream 54cm high bike, so what can you do until your child is big enough to get on a 26" wheel bicycle.
Well, there are two choices an all around mountain bike that is right for most kids and a more specialized road bike for going faster but not all terrain:
Specialized HotRock. Specialized probably makes the largest line of bicycles in this range. At the top end is the $600 full suspension A1 FSR. This is a little overkill as most kids won't need rear suspension, but the reasonable one is the HOtrock A1 FS 24, which uses 24" wheels and is a hardtail and costs about $350. Decent compoennts too and it is aluminum.
IBEX Vantage JayVee. Road bikes are much more difficult and smaller market. There are probably 10x more mountain bikes sold since a road bike implies that you are going to be well...on the road. We got this Ibex for $640 on the Internet. It is a nice bike although quite expensive. The other hand, the components are great and it will last well into teenage years because it uses 650c rims, it actually has pretty much standard components. Also, it has Shimano Tiagra shifters which are great and easy to use. All in all, at 18 pounds, if you have someone who likes biking and would really use it, it is a good value.
You do have to assemble it, but that shouldn't be too hard is you are reasonably mechanical. Ours shifted fast and was great, although the front brake caliper was a little bent.