What are some of the best kids (8-15) triathlon bikes for racing?
The one that fits.
You've got a wide range there. I don't know of a tri-bike that's going to fit an 8 year old. By 15 the kid should be ready for an adult bike. Somewhere in between an extra small road bike is going to be the best option. AFAIK, 43cm is as small as you'll get. A search for 43 cm road bike will get you on your way.