This happened here in the UK and the child molester, now in jail, didn't stop himself. Eventually one of his young victims grew up and subsequently reporting his molestations and rapes. Following this the man was arrested, hauled through the courts and the press and hailed of a monster.
http://www.silentlambs.org/georgecockerillukmolester.htm
This man was the person in charge of checking child abuse within the congreation and carried on his assualts and rapes for 40 years until stopped. It would appear that some people who abuse kids are unstoppable without the police being involved. It would also appear that some who have paedophile thinking do not go on to attack kids but refrain themselves and possibly Jehovah will accept their non-activity in paedophilia as proof they can control themselves but I would bet this is the least likely of cases.
For the protection of the kids involved and the future victims of this crime someone in some authority must be informed but, as was discovered here in the UK, you simply cannot trust everybody. It took 40 years to unravel the assualts on kids here so potentially the JW organisation is not the most reliable place to report this since the desire to keep things under the carpet and out of the media may be too great.
Since I was at the receiving end of this kind of abuse by a completely different organisation as a child my opinion is that I 'would be marching into my local police station and reporting all of the details to them' but for the fact that it was a social worker who was involved when I was young. Trying to get any authority to listen and act is a task in itself since I found out to my own cost that NO group or organisation or authority can prevent paedophiles carrying out their acts nor properly protect our kids. Which ever way you go NO-ONE can be truly relied on to help as it simply takes a lack of tangible proof and reasonable sounding denial by the paedophile to prevent this ever going further.
However, the one person who CAN be relied on in this matter will be the mother of the endangered kids. Take care in how she is told, if you tell her, so that she doesn't suspect your motives of being unreliable. It is a child's mother who must take the ultimate responsibility for her partner and her kids and it would be or great use to her to know. If her kids were later abused by this person and the Creator knows you knew of the danger then she would have to right to ask for the reasons for your hesitation in offering this information to her.
I would definately report the elders of the congregation to the authorities. THEY are compounding the dangers to these kids with their notions of not bringing Jehovah's name into disrepute here. And their inactivity is unforgivable. In this regard they will have to answer to Jehovah himself. Our Creator created all of this universe based on love and joy of sharing it with everyone and everything.
Paedophiles don't bring joy, nor does anyone who sits on this kind of information. Jehovahis not a God if hidden shame, He is a god of openness and protection and although I cannot speak for Jehovah as a parent I can understand how I would react. I would tell the prospective partner with great urgency. Being raised infoster homes and childrens' homes I know that having no parents to protect me was the thing that lead to my being abused so please, please, please inform someone.