Hi there. So, when you say 'diaper cream' are you talking about cream you put on her for say diaper rash? I would keep moving her hand. Kiddos do find their genitals I guess and they even masturbate. But as she gets older and older, the rule of keeping hands to self (no one touches you, you don't touch anyone else) and doing private things in private becomes important to keep stressing to them.
With regards to your husband, that's hard. How old was he when this happened and how long ago was it? There is also a pretty big difference between 21 and 15 . . . but yes, that is a different scenario than having any type of sexual attraction or touching with a baby. I would also be leery of the 'day care workers'.
Try to make a huge deal of it. Once something 'feels good' kids may keep doing it, especially babies that have no idea. But again, not sure what you mean by diaper cream.