Different DNS server for guest WiFi

And you could also read this thread, which creates a Safe WiFi for Kids on separate network (of course you don't need all these bells and whistles probably) - And you might need to isolate the "kids" zone as I'm not sure that this "kids" WiFi has blocked access to LAN:

I've not used this thread to configure my guest WiFi. I used the procedure from OpenWrt initially to create it a while ago:
https://wiki.openwrt.org/doc/recipes/guest-wlan-webinterface