At that distance he would not be a threat to you. In the exact situation you mention he would be a threat to the child, and most if not all states have statute covering use of force in the defense of others. You may want to look that up for your jurisdiction. What level of force will be dependent on the level of threat.
Sex offender registries are most usually compiled by the sheriff and maintained by the state police. I would ask the sheriff's department why this guy is not on the regular listing. Either there is some legitimate reason or something is wrong. In either case you will have learned something that may prove useful at some other point.
"A Liberal is someone who doesn't care what you do, as long as it's mandatory". - Charles Krauthammer