I would not get your daughter another single shot or pump gun... The recoil is atrocious, especially shooting slugs. It's a good way to turn her off from practicing and hunting altogether. I would opt instead for a used remington 1100 with a slug barrel. Take her and the gun to a gunsmith and have the stock cut down to fit. A cheekpiece is a good idea, as well. I have the leather ones that lace down on my 870, 11-87, and Encore muzzleloader.
Of course, you could follow Flintlock's suggestion and get her a muzzleloader. Lighter recoil, way more accurate, better ballistics, and really not much more expenditure than a shotgun and slugs. I'm still kicking myself for not getting into blackpowder sooner than I did. It's not at all hard to learn and it's only as esoteric as you want it to be.