You can fit a larger objective scope on an AR without needing a taller mount. Otherwise it's the same kind of scope you may already have for a coyote/varmint rifle. People have different ideas and preferences.
I have an AR with a Burris MTAC 1-4x. It stays on 4x when my daughter kills deer with it. She likes 4x or 6x. A Burris RT-6 would have been a better choice, maybe. That's what I'd get now along that line.
My son was killing his, usually, with a 1.5-6x40 on a 6.5 Grendel AR. It was set on 1.5x or 4x, depending. He likes less magnification, but more is brighter. I recently put a Meopta MeoSport 3-15x50 on that gun. No blood on that scope yet.
The common factors with the scopes referenced are ease of getting behind them and an illuminated reticle that turns down very dim. I find the kids are not as able to make a shot in very low light without a red dot on there. "Skill issue!"
Also with small kids you don't want short eye relief. 3.5" minimum and 4" is better. The recoil of even a 223 can really move those skinny shoulders back.