You're absolutely right havingamoraldilemma! His actions are HIS responsibility alone.
You are being very brave!!
Having a parent prosecuted for anything can be disruptive... But this is NOT op's responsibility, and should not come into her decision .. Also she is protecting HIS child and any child who comes into contact with him.
Regarding whether he is prosecuted or not... As there at least 6 of you, you know about.. Would /have any others come forward do you know?
Obviously like Savile case.. It is the intensity and the pattern of the behaviour that is important. I don't think there is much doubt that Savile was a prolific attacker, across age groups, gender and undertook opportunistic attacks and longer-term grooming, leading to attacks.
Also, someone doesnt have to be prosecuted for soc services action. Sometimes, esp with historic abuse, a person wont be charged as there is not a good prospect of conviction. (obviously they can still be guilty though...). The sexual offences laws really arent particularly good or useful for child protection, if only in terms of delays, decisions to prosecute, and the difficulties with securing a conviction. As of course it is in the nature of these offences, there are rarely witnesses, or physical evidence. (isnt the conviction rate in rape cases about 6 per cent??).
However, outside of the course, the child agencies (eg NSPCC) can do a 'finding of fact', this is below the criminal level of proof needed of 'beyond reasonable doubt'. This works on the balance of probabililities level of proof. (more like civil courts).
As a result, child protection agencies can intervene with the family to ensure kids are safe.
This is what happened many times when I was working - any more recent SW can please correct if this has changed!
Have courage and faith!!