I know of two cases.
One was in my congregation, used to read at the Tuesday book study, and always went out on field service with the elder who took the book study.
I joined the JWs at 13 but didn't find out he was a convicted paedophile until I was 18. I was in the same book study as him for my first two years of association without knowing he was a convicted child abuser.
Apparently, he had been disfellowshipped twice. After the first time, a long time ago, he went to prison for a spell.
He got DF'd again much later on in 1997 - not for new offences but because he'd covered up half of the old offences.
The other I knew of was the uncle of my best mate. My best mate, his brothers, sisters and parents were in my congregation from when I was 15. His dad was an elder. The elder's brother was the paedophile and he wasn't in my congregation. In fact, I never met him.
I remember talking to my mate's dad (he had stepped down by that time, over something completely different).
He was saying that some kids flirt with paedophiles and even want sex. I remember arguing against him, saying that nothing excuses child sex abuse and that the law states that children cannot give consent, in other words they're too young to have sex.
Theses two are the only ones that I knew of.