Are you receiving a message stating "Authentication Failed"?

Authentication failed indicates that Apple is not accepting the password entered.  
This Apple ID may have been locked by Apple for reasons that are not related to My Mobile Watchdog.  You must reset this Apple ID with iCloud before you will be able to update information from this device.  After resetting your password with Apple, please return to the parent dashboard to enter the new password.

You can reset your child's iCloud password  at the link below.