I have written why I say this before-
But I would not get a rifle for anyone, 9yo or otherwise, that they can’t, and don’t handle completely on their own. Load it, carry it, shoot it, unload it. Smooth actions without “hinks”, detachable mags, small size, lightweight, and little recoil( sub 10 ft-lbs). In that vain Tikka T3x Compact in 223, 243, 6.5 Creedmoor is probably the easiest and best option when viewed as a complete picture, though the Creedmoor will almost certainly recoil too much for volume shooting- and they need to shoot a lot. For that reason the 223 T3 with 1/8” twist is an awesome option. Zero issues on deer, bear, and even elk with the right bullets.
If not a T3 compact, then the Howa Mini 6.5 Grendel LW with stock mod is really good. The magazine catch can be shortened easily in about two minutes to eliminate what rgroves mentioned. With a stock mod/slimming it’s a true lightweight. They generally shoot very well, have low enough recoil to allow even the most recoil shy person a fun day blasting, factory ammo is really good and inexpensive, and it kills just fine way past where most people will even try.
Slimmed Howa Mini-
Weight with heavy rings, bases, and 19oz scope-