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.
For most on here there is a dream trip floating up around in their heads but there is a question of how to pay for it. This has been a problem for all hunters that wish for that elusive moose, bear, goat, sheep, elk, deer, or any other hunt where you are required to hire a guide or you choose to hire one for that hunt of a lifetime.
First off I am not a financial planner I am just a hunter that learned a long time ago that even with a great job there are going to be some...