I agree that you should go to the police, with evidence in hand if possible. His fiancee should also know, but there's a good chance she won't believe it, as women are very protective of their men, especially before they know them well.
The neighborhood where he lives and where he will live after marriage should also know. You could mail copies of his photo and a warning to those who live or will live close by so they can be on the lookout for any signs the children might have been abused. The children's teachers and school officials should also be notified.
I would also notify Child Protective Services and try to get a social worker assigned to monitor the situation.
This is a serious matter. Those children will be scarred for life if that psychiatrist is wrong, and he probably is.
Regards,
SandraC