thats a lot of muzzle blast for a kid to deal with. i mean.. lots of powder, small bore... i can see how a kid might be bothered by it. throw it in a light youth chassis... it probably does have a fair kick to it for an 11yr old.
if you're gonna do anything, get it threaded and let her try to shoot it suppressed? that might take the crack out of it enough for her to enjoy it. Maybe download it with a h4895 reduced load as well. you can download h4985 by up to 60% of max published load for any bullet/cartridge combo for maximum recoil reductionhttps://imrpowder.com/wp-content/uploads/2016/11/h4895-reduced-rifle-loads.pdf
or just get her something thats not so snappy, like an arc or a grendel. maybe even a 350 legend. easy recoiling, not a ton of extra muzzle blast, etc.
any of those three should be able to kill a deer comfortably and be more recoil friendly like the 223 is compared to the 243.
or just get some more 75gr sp's and let her keep hunting with the 223 for now. If she's comfortable with that - go with it. she's already made a successful kill with it.
whatever you do - dont ruin the kid on shooting by forcing her to use a gun shes not comfortable firing. aint nothing gonna turn a kid off on shooting sports faster than that.