Fuji makes a nice 43cm steel track bike that comes complete with 650c wheels. If you want to move up in price, Harry Havnoonian
makes a variety of high-quality bikes to fit smaller/junior riders. Also, you can check out the vintage selection at Mt. Airy Bicycles
- they have (or at least have had) a number of junior-sized track bikes on their list.
Finding a small-sized road frame that is "worth" converting isn't easy. Kids bikes aren't generally made out of decent tubing - a lot of it is really heavy single-butted stuff. Although I did recently score a really nice tiny Nakamura frame made out of Ishiwata 019. It's pink and was set up with a 700c rear and 24" front. Bizarre, but kind of awesome. I've also seen some quite small Nishikis, but finding stuff under 47cm is quite hard.
Also, you might want to PM Turd from this board; last I heard she had a junior-sized, lugged track frame (a 42 or 43) that she was thinking about selling.