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've spent countless hours either on my hands and knees or bent over, messing up my back, looking for blood. One such instance I remember most was a hog hunt a few years ago in Mississippi. I had never shot a hog so when I poked a hole in this guy; I was determined to find him. Wearing snake leggings, I crawled on my hands and knees through the briars and the brambles and bushes where a rabbit wouldn't go. I found a spot of blood here and there but never the pig. And lucky for me, I didn't find...