I have 2 other 4 port wired routers connected to it now. They are just REALLY cheap routers and are mostly just 4 way splitters.
Are you running the routers as switches (DHCP disabled, WAN port not connected) or are you "double NATing" (DHCP active, WAN port connected, different private subnets for each router)?
When you run a router as a switch, the parental control features will not work. If you're "double NATing", you need to lock away the routers else they can simply bypass parental control by plugging their pc into another router.
Usually when you use your own router you set the ISP router in bridge mode to avoid "double NATing", but that still leave the connecting directly to the ISP router loophole.
I would suggest that you check with your ISP and see if they have a router that includes parental control options, it would be the best solution. Of course they can always reset the modem but that would leave telltales.
My kids know how to pretty much bypass anything. Only way I could truly stop them would be to turn off the breaker for the electricity. lol. They have been defeating my attempts at parental controls for many years. I swear they teach it in schools now.
Even if they know what to do to bypass parental control, they shouldn't be able to touch Window's parental control or any hardware/network settings without Administrator privileges. You may want to consider changing their user accounts to "Users" and removing them from the "Administrators" group (Remember to first create an Admin account for yourself with a strong passphrase.)