Lets see....you are wanting to get a 12 yr old off to a good start with his first rifle. He probably has very short arms and will need something that has a 12" length of pull or close to it, otherwise, he will not be able to see through the scope.
His arms will not be strong enough to probably hold up a 8 1/2 lb rifle.
He will probably weigh in the neighborhood of 80 lbs or so.
I tried a flock of rifles with my little nephews, and it is tough. If it is too heavy, they can't get up on the scope, it kicks too much, you will just be inviting them to do something that they will have no confidence (or fun) in doing.
For their first rifle, just make it fun for them. If it is fun, it will build the right mind set for them and they will want to shoot it a lot.
You said that you hate muzzle breaks. Well, the cartridges that are not going to kick day lights out of them are few that can be used for big game. With all my nephews, the Model 7 Remington in 243 loaded with 100g bullets was way too much recoil for them!
What worked for them was the TC Contender with a 30/30 barrel on it and another 7/30 waters. I bought an extra stock, cut the stock for length, and saved the factory stock for later. My nephew killed two does, a buck, a bob cat and three hogs with it on his first year of hunting, he was 6 and was sitting in my lap in order to be able to shoot off the rest in the deer stand we had built. The 30/30 in a TC kicks very little and the 7/30 waters kicks less.
The little 6 year old could handle the Thompson Contender extremely well with it's very light weight and short configuration.
Another option that did not work for my 6 year old's brother that was 12 at the time. I put together a model 7 in 22/250 for him and planned to shoot the 60g nosler partitions. The gun was too long and cumbersome, even though I had found a wood stock and cut the stock to length for him. He preferred the TC 30/30, so I put together a 7/30 Waters for him...he handled that rifle very well.
Lots of guys here giving advise have never worked with kids. Issues for kids are the same as they are for adults which are, gun fit first and foremost, scope choice with adequate eye relief, trigger pull(critical for small hands and fingers), ability to grip and hold the gun comfortably, and RECOIL.
Just my 2 Cents