We cannot protect our children all of the time. As much as we'd like to, we just can't. We can do everything in our power to keep them safe, but then we have to try and make sure that we don't suffocate them at the same time. Parenting is always a tricky balance, getting it just right is nearly impossible. And even if we do everything right, everything in our power to protect them, things can still go wrong.
I clearly remember reading a journalist's interview with a paedophile, he was asked what could have prevented him from raping and killing a little girl and he said "perhaps if she was strapped to her mother, but then I'd probably have killed the mother too". Chilling stuff.
I let my 7yo dd cross our quiet road to play in the school playground by herself knowing full well that she might be hit by a car crossing the road, or she might be snatched as she plays by herself, in fact I can think of 100 things that might happen to her. But I know that she also needs to be able to have the freedom to enjoy her childhood. I don't want to take that away from her. So I see it as a calculated risk, as she gets older the risks will probably get greater as she demands more and more freedom.
To the OP I would say that you now know your fil has these tendencies, so you and everyone in his family will be watching him like a hawk. You are in a better position than if you didn't know. How many times are our children exposed to paedophiles that we don't know about? At least you have that knowledge and he knows that you know, which makes your dd safer.
But for now, just listen, watch and wait. I doubt very much that your dh will let your fil have unsupervised access to your dd once she is born. He will fall head over heels in love with her, he will experience love he never thought possible, to the extent that he would die for her. So just wait until she is born and do not be drawn into future scenarios just now.