Thanks for all the suggestions, guys. I got a cheap set of mirror screen protectors off amazon because that's what she wanted.
I may yet try and fix the screen. It certainly seems to be a very different scenario from when she broke my original 2G iPhone screen. Back then, the only guide on how to replace it was by a guy who spent 8 hours removing the original screen with a razor blade and had loads of comments at the bottom from people who broke the touchscreen sensor trying to imitate his method. I spent £30 on a replacement screen from China and then never tried to put it on.
Nevertheless, the crack is only in the corner, so the screen is 85% fine. Its just that it hurts to run your finger over that bit. And I'd rather live with it than go to the trouble of fixing it, considering her phenomenal track record:
i) borrowed my first iPhone for a day and smashed it by 9am
ii) got handed down my 2nd iPhone and lost it before I'd even gotten around to getting it unlocked and putting a simcard in it
iii) got handed down my 3rd iPhone and broke the screen, albeit having managed an encouraging 3 months of heavy use without incident
So fixing it will probably be more of a Christmas gift...