I'm so sorry you're going through this. It's a horrible thing to have to deal with and I don't think it's ever one of those things anyone anticipates will ever happen to them or to anyone they know. We had a very similar situation, but with a family member (well, married partner of a family member). Though in his case, he was arrested and convicted of sexual assault (against a child under 12) and even worse, quite a few family members went to great lengths (for years!!) to hide this information from us so that he would be able to have access to our dc. Fortunately, we have no reason to believe our children ever came to any harm. They were never unsupervised with him (though, of course, abuse can still happen in plain sight and I acknowledge that), but though he was around them regularly, never in a capacity that would have allowed any harm to come to them. Equally they were also not his 'target age' (pre-teen, our dc were baby/toddler stage), but never the less, it was horrible. Made even worse because of the deceit and the extent that people in our lives that we trusted went to to hide this from us and make sure we never found out and could block further contact with our dc.
Fortunately, we did find out as we eventually had to be notified as we were planning a holiday to stay with family (including said family member and her partner) and it came out that in order to stay over in the same house (we never had before, which was weird, but I actually went years thinking it's because they hated me for some reason!), we would need to meet with the police to consent to the overnight stay as a condition of being on the sexual offenders register. If not for that slip up (they thought his 10 years would be up before the holiday, which is why they planned it, but turns out the dates got messed up and his was still under restrictions on those dates), we probably never would have known. We've since faced years of gaslighting and manipulation by several family members for not just 'being cool' with his past and getting over it and allowing him contact with our kids (hell no!!). It's fractured our whole family and though we do have a lot of support from many people in our lives, others have been downright nasty and abusive to us because we've not just buried our heads in the sand.
Like you, it's made me completely question my own judgement. I trusted these people and they intentionally misled me and manipulated me to facilitate his contact with our children. They still would be if we hadn't cut ties. We've become the black sheep, the ones who are 'too precious' about their children, who can't see what a victim his is and how much all of this is ruined his life and us doing this is just one more 'injustice'. It's truly like living in the twilight zone some days. How did I not see that these people we trusted would do this to us? It's made me so paranoid about everyone else too.
My stepdad isn't close to his own biological children (bad divorce, caused by his wife, but which affected his teenage daughters at the time quite badly). I've gone as far as to panic and distance myself from him and my mum at times worrying that maybe he abused his children too? Is that why they don't speak to him? He seems so lovely and kind, but how do you ever know? It's awful. (I should add, I think my stepdad is great and don't actually think he would hurt a fly, but this stuff really messes with your mind). I've had panic attacks about kindly old men coming up to my dc on the street and talking with them. How do you know anyone's motives after something like this? It's a mindfuck.
My advice would be to talk about it, keep talking about it, make sure everyone in your life who has had contact with him gets the information you've gotten so it doesn't stay a secret, and give it time. It's been several years since this all erupted for us and it's better now. I'm still angry some days. Said family member is still there, still aggressive and occasionally sends abusive texts to my dh (we've considered a non-harrassment order, but not pursued it yet), we still can only attend limited family events, but I don't have the anxiety I used to have and things are much better. Hang in there. I know it's shit. 