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 could see the small band of mule deer picking their way towards me through the sagebrush 800 plus yards away. With my 10 power binoculars I spotted a legal buck bringing up the tail. I dropped down and crouched behind a large chunk of sage just waiting for them to pass within range. Unfortunately, they stopped and began milling around before finally settling down, with a few of them laying down right out in the open. They were not going to come to me; if I wanted any closer, I'd...