From time to time I see junior sized track bikes on Ebay. These are 700c or 650c wheeled track bikes with frame sizes in the 46 - 49 cm range.
Also, take a look at the new Specialized Langster fixed gear/track bike. Their 50cm frame has a seat tube length of only 43cm with sloping top tube. That'll probably give you plenty of standover height even with 700c wheels.

George