What are the options?
1 - Change wifi password.
2 - Fixed IP
Before this step, you must first log your phone to wifi SSID and then set your phone (tablet) so that the phone does not use a random MAC, but a MAC device.
(If the phone logs on to another wifi network it will still use a random MAC address, the default MAC will be used in your home wifi)
In the next step … reduce the range of DHCP max leases so that the router does not assign any IP without a defined MAC:
** DHCP start 192.168.2.101
** DHCP max leases = 3
Number static leases is equall max leases:
Pixel4a-Jarda 58: 24: 29: 68: xx: xx - 192.168.2.101
Kaon-102 90: F8: 91: 9F: xx: xx - 192.168.2.102
Pixel4a-Jirina 58: 24: 29: 68: xx: xx - 192.168.2.103
MAC cloning is possible, but it is not so trivial
3 - modify the firewall rules in Luci (iptables) but I have no idea how else to identify children’s devices … I’m not an expert on firewalls.