.223 is not my preference, but the AR is the only gun that my 6 yo daughter could shoot last year due to the length of pull, and she wanted to go. She proved worthy of making good shots so we headed out with the Caldwell field pod.
Her and my 9 yo son killed a doe the same morning with the same gun. The deer turned on her as she shot and she hit it in the ham. It demolished the ham and turned it to mush. Luckily it hit the artery, and the deer didn't go far. This was about a 75 yard shot. 2 more came through a few minutes later, and my son wanted to try the AR on one as well instead of his 7mm-08. It was about a 90 yard broad side shot. The deer jumped as high in the air as I have ever seen one and rocketed towards us as it made it about 50 yards coming straight towards and then turned and fell. I could see the blood pumping out of it.
We were shooting 62 grain Federal Fusion. Again, they are not my preferred round, but I wouldn't hesitate to let them use it again and didn't have a problem using them either after seeing what they would do based on some of the reading that I had done here.
My daughter should be big enough this year to use a youth model 30/30 with a red dot that my son killed his first 7 deer with. The red dot is awesome for a kid having a hard time finding a deer in the scope. He has gotten older and moved on to a scope with no problems, but my daughter like to have never found the deer in the scope on the AR with it standing in the middle of a wide open field. We had practiced as well at home and in the field with no problems. A cat came through the morning before in the same spot as the deer, and she got right on it. (We didn't shoot it by the way. I just wanted to see if she could get on something that small and moving. We did see it a hunt before, and she asked if she could shoot it. :super: I didn't let her but was surprised that she would even consider.)
If it's the only gun that they can use due to recoil, LOP, etc., I wouldn't hesitate with the right round and good practice with the kid.