Or what has already happened with other kids/at other times! I would report it to the library, the play group, AND the police! With the police, just call their non-emergency line and they will send an officer to your house so you can make a report. Or you can stop in at a police station to do so if you are more comfortable with that. The reason for making a report to the police is if this is a pattern of behavior that others have already reported, or report in the future, the more reports they get, the better they can describe and find the guy. This is not a minor incident that you are describing!