Certain felons (such as those convicted of certain sex crimes) cannot be near children, and no felon may voluteer in a public school activity (such a reading to children or going on a field trip), but no Florida law prevents convicted felons from visiting their children at school.
If you wish to stop visitation between a father and his daughter (a dubious thing to do in most circumstances), the mother should go to court and seek sole custody. If the judge won't award sole custody, then perhaps it's time for the father to have a chance to make up for his awful mistake. Your granddaughter may not like the visits, but at her age, how would she know? Sometimes, a bad dad is better than no dad and, given a chance, maybe he'll turn out to be a good dad.