We have a 6.5 CM and a 7mm-08 that our 3 boys have used on several deer and elk.
Assuming you reload.....if I had to pick one, it would be the 7mm-08, especially with elk on the menu. The 6.5 CM was better for when they were 10 and under because I could load up 100g BTs at 2700 FPS, which were very mild. Otherwise, the high BC 6.5 bullets don't give you much of an advantage at ranges most kids are capable of shooting at. The 6.5 was perfect for deer, though.
The 6.5 isn't very forgiving on elk with anything other than a boiler room shot, which is certainly possible with kids and field shooting conditions. The 7mm-08 with 140 ABs is a solid elk setup, kid or adult, and we usually get exit holes.
