With a child of my own interested in running a marathon, I researched marathon training programs for youth in the US (there are some!), injury rates and how these compare to high school running programs.
If you see my other comment the volume is actually where the risk lies. There is generally no problem with kids doing close to 100% VO^2^max or HR for extended periods. They'll just get pooped and quit before they can do any damage. It's the repetitive stress of 10s of thousands of foot strikes that can put immature tissues at risk.
Notably bones aren't fully calcified and growth plates are open at the end of the long bones. There isn't really a good understanding of what stress and exercise induced inflammation do to developing tissues...at least that I'm aware of.
Edited for autocorrect. 😡