Well I'd start by not having the main character go to an online erotic story site. Always seems such a forced and self-referential thing. I'd have the girl start misbehaving to get punished more often, and experimenting by herself, with things gradually (or not so gradually) either leading to a parent catching her in the act and getting a confession from her (which is naturally where the real fun just begins) or suspecting what she's up to and deciding to gradually up the ante to see how far the daughter's willing to go.