Hi, sounds like your kid has used a spoofing program/app to change the mac address on his device/s or has access to your router interface to change/undo your settings......

Possible solution assuming he/she is using wireless connections

1. Change your password on the router to eliminate access.
2. Find his IP addresses Device name and mac address on the device/s an add them to parental controls section making sure to enable parental controls as well then save it, if more than 1 device you`ll need to make more than 1 entry........now it should have MAC/IP/NAME and you can now set access times.

hope it helps.