Here's one I made for a 10 year old girl. The length of pull is 11 inches.

He parents were well off, and wanted a "family treasure" for her to start with.
It's a 45 caliber. They are into muzzleloaders (obviously)

The principal of making every shot count is something a muzzleloader will teach a kid VERY well. The little thing is very accurate. At 40 yds I could hit quarters with it from a rest.