You can find the MAC of the Xbox in the network settings, and the PC should show it's 'physical' address (i.e. the Xbox One, the PC, the smartphones etc). What you need to do is: Create a list of MAC addresses used in your home (e.g. I don't know about your network setup, but you could try a combination of using MAC black-/whitelisting, which will take a bit of work. I'm sorry to hear that you have trouble with your sneaky kid getting around the Circle parental controls.
