at 20-25 yrs it SHOULD be enough to kill a turkey, but its not rocket of energy past that point
it can be a combo of things, from a tuff turkey to kill, or simply NOT hitting any vitals, turkeys are pretty tuff birds.

MY suggestion might be to put some targets up from the like set up shooting conditions and see if he can again make the same shot, like if shooting from a blind OFF hand or??

also, can try taking an old phone book out as a target and see how far into it the bb's go?

I know there NOT common to find, but a good 28 gauge , pump, or more rarer a single shot 28 gauge is an awesome kids shotgun, lots of power, not much recoil, there light and nimble, just what a smaller shooter needs
and if your OK with spending a little $$ a semi is a gun, that once they kid is done you will find you WANT to use for everything you once used a 12 GA for, MINUS< maybe waterfowl
28 ga's are super pheasant guns, a caliber/gauge so few ever use??

I'm NOT a fan of 410's to be honest!
but they can kill , just make it harder than needed ,IMO> better tools out there for the job!