Do frequent nightmares indicate some kind of repressed child molestation?

NO. No, nightmares, even very frequent ones, when taken alone are not specific at all for previous abuse. What that means is that while some people who are abused might get nightmares, most people who have frequent nightmares are not abused. Therefore, frequent nightmares cannot be taken to be a good indicator for previous abuse experience.
No. Usually not. Nightmares can be from child abuse but most kids with nightmares are perfectly healthy and safe, they just have nightmares. Talk to your child to see what is going on.
No. This is a "pop" claim that has led to witch hunts and devastating false accusations. If a "therapist" has told this to you, in the lack of any solid evidence that child abuse has taken place, report that individual to the professional supervisory board. If a child or adult seems troubled otherwise, get with an evidence-based therapist and talk -- understand the past, look to a brighter future.