One of them is called Life360. It allows you to "check in" with your children or even your spouse by using GPS capabilities.
The company says the app has 20 million subscribers. It also includes a "panic" button that sends out rapid alerts in case someone is in trouble. The app is free and available on iPhones and Android phones.
Another app out there is Footprints, also free in the Apple app store but you do have to pay to subscribe to the service. It allows you to set up "geofences" to keep your children in a certain area. If they hop over this virtual fence, you receive a notification.
If your child does not have a smartphone, you can purchase GPS trackers like AmberAlertGPS. You have to purchase devices and pay for a membership.
Obviously you shouldn't make your children feel as if they are under surveillance. It would be good to have a conversation if you decide to use one of these tracking apps.