How to deal with your little girl all grown up? Gently? Respectfully? How did you deal with her being 10? Same kid, new day...
The best way to stop a replication of the past, that I know of, is to tell the entire truth. I have watched the fallout from SO many parents who withheld or obscured the truth that it now appears to me that the very best way to get a child to have an unplanned teen pregnancy is to try to stop them finding out out sex, pregnancy, diseases, the function of their bodies, illicit drugs and (horrors) what mom and dad did way back when. That mystery surrounding the subjects creates a huge wealth of curiosity that the kids can't get satisfied in any way other than personal experience.
Kids who know the whole truth don't go looking for experiences to find the answers they already have. They, in my view, approach sexual relationships in a more thoughtful, personal and far more responsible and mature way than the ones who are essentially thinking 'man, if mom gets this wound up about it, it must be REALLY good!' (That's a direct quote from a friend who got pregnant at 17, 2 months into her university education, which stopped right there.)
If you're concerned about pregnancy before the pill kicks in (about 6 weeks after she starts taking it regularly) supply the child with a spermacide - gel, foam, doesn't matter what kind. That will increase the effectiveness of the condoms to nearly the Pill's level.