It might be best to let your actions speak for you over time instead...and be extremely respectful of the baby's father when he is in the mix. Though I agree that no man is going to be thrilled at the prospect of his child being around another man that regularly, but if you are respectful and supportive of his relationship with his child, then of course they would appreciate that you care for there child when you are around. Your relationship with the child will be what you both make of it regardless of the rest.

As jim said, is the dad around? If not, then just tell her how you feel. It takes a good man to raise another man's child