This is populated based on the Network Map and DHCP table in the router. Give the router a reboot first, then try rebooting each device which does not appear on the list.
One note about the Parental Controls feature is that it is known to slow the router down. It may be better to run Parental Control software on the devices themselves versus the router.
If you have created a rule selecting ALL of the devices on your network, attempting to create a second rule will show no devices. No saying this is right, wrong, desirable or not. This is how it works.
However since the first rule doesn't do what it looks like it would do, not sure anything in this catagory of code functions at all.