I presume that "detectable" means "detachable". Offhand, if you're talking new models, I am not sure which have detachable magazines. It has always been my preference but the rifles that have drop open floor plates are excellent as well. I think that Remington discontinued the rifles (ADL models) with internal magazines that do not open.
The .243 doesn't have a whole lot of recoil. If your niece is a tiny little girl the recoil might be annoying but if she is more solidly built I think she will be fine. If the rifle is scoped, and especially if it is wood so it has weight, the recoil shouldn't bother her much.
Also, perceived recoil can be highly subjective. If the gun weighs enough, and if you get it fitted to her, and add a good recoil pad, that will help considerably.