Ya, I think 8 is the max for this model router. You may need to manually limit what the kids use, by maybe keeping the device locked up for the period of time they can't be used. Try to figure out what devices need the management by the router and lower priority devices that would need to be managed however can't be by the router and would need external intervention.
The recourses on these routers are limited. They were based probably on the average home not having a ton of devices as some years back. Now as technology grows, house holds now have numerous devices.
Newer generation routers may have more recourses to allow for more device management. I'm still limited on what I have for a two person household.
What i the grand total of household devices in your home?