Here are some things I found when looking for information on statting out a child in GURPS.
This is a set of templates for children, based only on age, which I put together from posts starting
here.
It is in my own shorthand (mostly typical for the forum). If anything is unclear send me a PM and I will either edit the file or inform you directly.
This is something official which is specifically focused on height and weight for boys 2-20 years old.