Howie,
As far as the caliber of the rifles the kids on my sponsored hunts use, well it depends on the type of hunt. Depending upon the ranch and event sponsors we offer modern arm, muzzleloader and shotgun hunts. For any of these hunts the kids have a choice of bringing their own firearm or using one that I or one of my volunteers supply. What we try to do is make sure that the firearm used matches the youngsters shooting capabilities. After all, even the biggest, toughest 18 year old football players are sometimes recoil sensitive. Usually the calibers that we supply for modern arms run the gambit from .223, 6.5mm, .243, .7mm-08, .308 and .30-30. We also try to take into account the animal to be harvested and the terrain to be hunted. The worst thing that can happen is for the parent to bring an 85 lb. 12 year old young lady to a hunt with dads .338 magnum. If you have read my posts then you know that we always try to have some type of shooting exercise so that we can watch the youngsters shoot their chosen firearms before the hunting begins. This is when we can determine if some type of change in firearm needs to be made because, after all, our main goal is to provide an enjoyable and educational experience, not one in which they get scope bit or knocked about by heavy recoil. This would only cause them to want to quit hunting before they learned to enjoy it.
As with the modern arm hunts, for the muzzleloading hunts the youngsters can bring their own gun our use one of ours. We have both .45 and .50 caliber guns available in both traditional and various inline configurations. When we are limited by the landowner to the use of shotguns we can provide them with either a .20 or 12. gauge model. (Yes, some ranchers do require that we only use shotguns on their ranches. This can really make for interesting hunts as, even though most shotgun slugs can have the same effective range as some muzzleloading rifles, most people in my part of the country are just not used to using a shotgun for big game.
Hopefully this information will answer your question. If you ever need any more information of ideas please let me know.