A lot of manufacturers still sell junior size road bikes in Europe, and you can often find them in US. Among the choices:

Specialized Allez Junior with 24 inch wheels
Bianchi Jr 24 (in Celeste blue, naturally)
Felt F24
Giant TCR Junior
Colnago used to sell a junior racer with 21 inch wheels for even younger kids

These typically sell for <$1000 new, and often for around $250 when you find one on eBay.

I agree with Eden that a great option is one of the junior cyclocross bikes such as the Redline Conquest 24 or the REI Pulse.