kids
I've grown more and more fond of the .243 as a deer round for both kids and adults the more I shoot it. First week of Jan I killed a nice 8 pt out at 250yd plus with my Dad's .243 that came my way after his death. Bamaboy hunted it as a kid, killed 5-6 with it, and it has set idle for the most part since then, till this year, when I hunted it again. The 100 gr Nosler Partition sailed right thru, lopping off the top of the heart in the process.
I'm opposed to traditional 30-30 lever rifles for kids and newbies. Lowering the hammer to half-cock seems a complicated step for many, and many folks these days do not understand the correct process. The lever rifles with the safety address that problem, but I cannot warm up to them either, though they are no doubt "safer", at least in the eyes of the tort lawyers.
Bamaboy killed his first two whitetails with a .223 bolt rifle and quality bonded bullets. He was well prepared with me coaching, there is no doubt a proper bullet in .223, and picking shots, can take deer, but I really cannot in good faith suggest the .223 as a deer round, there are better choices of caliber.
Nor do I like the idea of starting a kid with an AR, magazine or not. That may be an appearance thing, and yes, I suppose I should get over it. I have an AR, and am considering building another in 6.5 Grendel exclusively to deer hunt with (probably won't happen). And if someone capable wants to hunt with an AR in the appropriate caliber, great. But somehow, junior with the AR doesn't ring true to me.
What does seem like a good idea is some type of bolt rifle in 6.5 Grendel, or as an alternative, 7.62 x 39mm. The Ruger American Ranch is available in either caliber these days and would be an extremely mild, but effective deer killer in either caliber. Affordable too. I hunt a Ruger lightweight M77 in the x39mm chambering and have no hesitation using it inside 150 yds on deer, and do so frequently. Similarly, the new Howa Mini-Action rifle in 6.5 Grendel and the featherweight barrel should be a joy to hunt as well, for a kid or an adult. They run $500 bucks or so if you can find them, but the LW version is feather like.
Weight is an issue with kids.....the weight of the firearm and their ability to manage it. When you get the firearm down to a weight that a 7-8 yr old can handle, a serious caliber like 7mm-08 is a real kicker as others have noted. Even in a full size 8-9 lb rifle, I would not start a kid on a 7mm-08.
Junior needs to be able to manage the rifle away from the bench or the shooting house rail. The single shot H&R rifles were good for that.