The 25 WSSM is about the equivilent to the 25-06 so recoil shouldn't be that bad. To say it was too strong for your 9 year old would depend on the child. It is not strong recoil but what other calibers has he shot with?
His age is not really the determining factor of his ability to accept recoil. I've been a competitive skeet shooter for years and have wanted to teach my brothers kids the sport. Most professional instructors (see Fred Missildine's book) will advise you not to start a child until he/she weighs 75 pounds. For skeet shooting that would be the min weight using a 20 ga autoloader to start. To much recoil to early develops bad habits...... the body will naturally avoid the sensation of pain. The kid may develop a flinch or a hesitation to pulling the trigger or something like that, all bad in shooting sports.
I'd use the same guide lines for rifles.
I have learned quite a bit over the past few years turkey hunting and the majority of it was how to get that tom within range. Every article you read is littered with talk of fighting purrs, flydown cackles and clucks. What they don't tell you is now that you've caught that tom's interest and he's coming on a string... now what? I have seen more blown or missed turkeys from lack of experience in this area than I can imagine. If you can't seal the deal than you may as well have left the...