My granddaughter will be 9 in a few weeks and I need help?

My son is a single Dad raising 2 daughters ages (almost) 9 and 6 and a boy who is 4. Their mother is totally unfit althought she does visitations. She is not a real mother to any of them. She is totally permisquius and certainly no role model for the kids. My son doesn't know how to approach the subject of . I'd like to talk to her but on a different level - about not just having , etc. but to wait until she truly loves someone, not to ruin her life with unwanted pregnancy, etc. Does anyone think is the right age for "the talk" and does anyone know of any resources from a Christian standpoint? Thank you.