Just create a separate SSID and let them use that. However, then you ensure that that SSID has something like AdGuardHome to do filtering on it and thus they get "kids restricted" internet. There is a opkg version available. There is also a manual install thread. You can also enforce safe search and other features.
(you can also adblock with it but i'm assuming what you want off the bat is kid safe network) You can do exceptions for adults via client management. Thus adults dont get filtered internet.
Ideally having AGH do your dhcp would be better but OpenWrt's dhcp is far more powerful and the AGH team are still refactoring their software to better handle more diverse setups. Thus the compromise and slightly more complex install procedure we use to do AGH filtering but keep the power of OpenWrt's dhcp.