You said yourself that you enjoyed the sex with her. Just because the relationship has gone sour, doesn't mean what you two did together sexually suddenly becomes bad too. You obviously have some pretty good memories of the sex that you would still masturbate to them. And they are also your most recent memories, therefore probably the strongest.
For years I had an ex-boyfriend with whom I'd had great sex - but I wouldn't go back with him to save the world - and my fantasies would turn to him even after subsequent boyfriends. (Fortunately, I have a much better/hotter replacement now.) Didn't mean I wanted him for real, it was just something to masturbate to that worked well at the time.
Anyway, just because you are fantasizing about her doesn't mean you are still in love with her and want her back. Just that the sex was good. (Unless of course, you are still in love with her and want her back, but that's not relevant to the fantasies. You can keep them separate in your analysis.)
As for your child, keep strong and don't let her get away with it - even if it means legal action. Hopefully your friends and family can see both sides to this bad situation, they are your friends and family, after all...