It would depend on the reason for the parent being on the sex offender registry. For instance, a child that is in my DDs class has a father who is a registered sex offender. I let my DD play over there all the time. Why? Because this guy was 18 dating a 15 year old and was charged with statutory rape, that was 12 years ago and the guy and the gf are now married with 4 kids. So, that really does make a difference. Now, if the guy fondled a prepubescent child, then heck no, I would not allow my child over there.
But, I would also trust my instincts.