Why not take the path of least resistance and buy a 243?
You don't know if the 11 year old will get bit by the rifle bug and become a reloader, which would be a big advantage if you choose to go with either the 257 Roberts or 6.5x55.
I'm like both the 257 Roberts and 6.5x55. I currently have 10 of the former and 11 of the later, but I think that both of them are much more useful for the shooter who reloadds and who has the foresight to plans ahead, stocking up on component brass when it is available.
Exactly on the 243 (Or *gasp* 6.5 CM).
