@ladykuga. This is how you do it.
MAC Address filter
For Virgin type 192.168.0.1 into web address (other ISP may same or 192.168.0.0)
Use password provided (usually provided on bottom of router, so change it if you have got smart kids)
Under security, select MAC Address Filtering, select MAC Address for equipment you want to limit access, set limits, save new settings.
May need to first look at connected devices to find MAC addresses for child’s equipment.
We have had Virgin Media, Orange/EE and is available on both ISP so expect it to be on others, just need to search for the setting.
It seems a fiddle to do it but once you have done this you can always control your dc access to their devices.
OP this is what you need to do to stop your child having the upper hand over you.