I think now the subject has been broached, if you don't want to go into too much detail you could just say that there are some bad people in the world and its daddy and my job to keep you safe.
You don't need to go into too much detail. Especially if she's young enough to be with you or your dh most of the time.
Recently my 2 boys were sharing a bath and ds1 touched ds2 willy messing about. Now I know it's completely innocent and he was only playing, but I did say to ds1 that he's not allowed to touch any willy apart from his own, and no one is allowed to touch his.
He also knows he's not allowed to run off while we're out and around corners where I can't see him etc
But I also make sure he knows he can speak to me about anything. That he shouldn't keep secrets from me and I want to know if by one hurts him, and he can tell me the truth about anything.
I've really laid it on thick that he must be truthful and honest and if something is making him sad, unhappy, worried etc he must tell me so I can make him happy again.
It's such a difficult subject for little ones and not one I enjoy but feel it is necessary unfortunately.