onmymind - at the end of the day as you say, you can only go by how you feel. it is important that you let your kids now what is acceptable and what isn't. your dd spends time with the dad, and even if at best the dad isn't iffy, she still needs to know for any adult.
If all of this is down to misunderstanding, the fact of the matter is dd spends time with the little girl who will tell her stuff and so your dd needs to know whats ok and whats not.
alot of abuse goes on for a long while before the child even knows that anything is wrong, if they have been exposed to it from a young age, they dont know that it isn't perfectly normal.
I am not from a legal background so cant say what is right to say, you cant not say anything in fear that it may discredit the child, children are sponges and get their knowledge from all diff sources.
In court cases the judge/ legal team need to acertain if the child kniws the diff between right and wrong so her saying she knows it's wrong cos you said or tv said or whoever said still means she know it's wrong.
another thing to think about is if something indeed is going on and you knew about it but did nothing 1, you'll feel awful, 2, the mother will want to know why you said nothing, 3, you'll want to make sure nothing like this happened to your dd as she stays in his care. and 4, if you did have to go to court and the dad really had done something, the defense will make out that the fact that you did nothing proves you had no reason to think anything was going on and that will help him.....
right i've waffled on far more than i had meant to.
oh yes, last point - as said before - you dont have to out rightly accuse the dad, you can just tell the mother and leave it in her hands.