Actually some perverts can be and are cured. Not many. But a few.
Demonising people doesn't help anything at all.
This is one of the most abhorrent forms of criminality because it's so utterly destructive. He is still a human being though.
I'm not sure exactly what you are asking here choccybaby. It has to be hardest for your husband because the children of people who have acted in this way are still their children. Many of them still love their Dad, including people who have been abused in ways that you certainly could never have imagined until you come across the fallout.
I think that it would be very understandable to distance yourself. If it's possible then I think that you need to support your husband in having the level of contact that he feels is right. It would be a very big thing to ask that he goes complete NC unless he himself wanted to; it would leave him with intensely difficult and conflicted feelings possibly for the rest of his life (another consequence of his father's crime :/ )
Regarding your children, well, it goes without saying that they could never be left alone with him. The biggest pragmatic problem now is that he is wanting to sweep it under the carpet and not to acknowledge how awfully he's behaved. Because of that I'd personally think that it would be wise to keep him well away from them, minimizing does not bode well.
Things can never be the same for any of you, FILs actions have altered the way everyone thinks of him forever. But I think decisions to go NC have to be thought about carefully and not done as a twitch reflex.