On your child's iPhone, go to Settings -> iCloud and log in using your Apple ID and Password.
If you do now have an Apple ID sign up for one for free at:
If you already see that an Apple ID is associated with the iOS device, ask your child to give you the password for the same or log out and enter your new Apple ID and password.
Once the iCloud login is done on the devices, make sure all the settings are turned on, except Messages. Messages must be off.
To track a regular GPS location, Find My iPhone service should be On as well. Go inside Backup and tap on Backup Now so you create an iCloud backup immediately.
The iOS will backup your device automatically every day when it is connected to WiFi, charging and the screen is locked. It may take up to 24 hours for the recent data to show up in your account.