It's a tough situation.

On one hand, he's a menace to society, and should never be allowed to be in contact with children, and for the safety of the children, he should be monitored.

However, on the other hand, it could be argued that since he has served his sentence, he has paid the penalty for his crimes, and should be completely rehabilitated by now, and given an extra chance. Does his sentence end when he leaves the prison? Is it the duty of the prison to rehabilitate prisoners in situations like this, or just acknowledge them as lost causes?

Given that he has already re-offended (and spent over 20 years in prison), his chances of being rehabilitated are slim, but I'd suggest to monitor him for a period of time (a year or two, perhaps), and if it's of the opinion that he has reformed, release him as a normal member of society. If he still hasn't reformed, continue monitoring him until he has. If he re-offends again, then life, no parole.