That is really cool. I know I could've never passed on any animal the first few seasons I hunted. Congratulations to your son and family.
Since the rule change just happened this year, was Timothy already shooting big game rifles? Granted, he slayed a deer with the muzzleloader two years prior. With the new rules and children of my own not too far off, I am curious as to when parents are moving beyond the .22 caliber. Obviously it depends on the individual child.
Timothy had extensive experience with the .22. Moving to the centerfire was a real big step for him since the smallest centerfire I own is a .270 that kicks harder than my 30-06. I shortened the factory stock on my '06 about an inch so it fit them better.
In practice, Olivia was definitely more comfortable with the recoil than Timothy was. I spent a lot more time with him dry-firing the rifle than with live rounds. I knew that when the moment of truth came on game, recoil wouldn't be an issue but he was having trouble overcoming his fear of the unknown at the beginning.
They both did well for the size of the gun they were shooting. Having said that, before any more seasons roll around they will be shooting a different gun. Still undecided whether a .243 or a .260 is going to join my other guns or whether the 30-06 is going to find a new identity as a 25-06.
Key for both of them in my shooting strategy, was controlled shots off of bipods. I want them to be confident that what they are shooting at is going to die before they ever pull the trigger.
Olivia firmly believes that when she pulls the trigger, the animal is going to die. At some point reality is going to teach her a hard lesson like it does all of us, but I'm sure not going to shake her confidence. All her shooting in target practice and at game reinforced her confidence.
Timothy has been more hesitant at what ranges he is confident at. I'm sure the two year age difference has some bearing on that. The 100 yard bang/flop he pulled off on that doe has him feeling pretty good though.

He shot a friends .260 and said it didn't kick much at all, so I'm really looking forward to seeing him getting a bunch of rounds down range before bear season.