I have used Family Watchdog for years. It is a free service that helps you locate registered sex offenders that live near you. Of course I realize this is not foolproof and we must still carefully watch our children, but it is a wonderful tool. I even use their site to look up their grandparents' or friends' neighborhoods where my kids play outside often. If my children attended school, I would look up that area, too.
I have recently learned about www.sexualoffenders.com,which not only allows you to look up sexual predators in your area but also allows you to sign up for email updates if a sexual predator moves into your neighborhood.
I hope these tips are helpful in keeping your children extra safe.