You can buy a custom stock with the LOP you want or find a used youth stock. Personally my son is 6 and doesn't shoulder the rifle, I hold the back of it for him or have it mounted on a tripod. He is shooting a 6.5 Creedmoor with a suppressor.
Last year he was shooting a 300 Blackout AR, so I could adjust the LOP for him, but we swapped this year because we are going to Africa in june and can't take the AR for him to hunt with.
If I was just hunting in the states I would build an AR in 6.5 Grendel or 300 Blackout with a carbine stock and let her have at it. I ran 10 round magazines with one in the chamber and one in the magazine when he was hunting with the AR