Correct me if I am wrong, most typical routers (Asus, Linksys, etc..) doesn't give you the option to block by IP. These routers only give you the option to block by URLs. If you have teenagers, it is a matter of time before they figure out the backdoor around the basic parental controls.
The one parental control I use, as mentioned above, is time control. It allows me to do this on a per-device basis, by device Mac address. There is no access for any kid devices at night. Now, the oldest one could figure out that cellular data still works then, but she likes sleep too much. There is an emulator for the UI at this link.
You mentioned, you have have a TC connected to the Asus. How did you correctly configure the TC to work with the Asus? Are you using TM on the TC?
The main reason for getting the TC was for TM. As Bob mentions, it only backs up Macs, not the iThings.
As I wrote above, the TC WAN port connects to a LAN port on the ASUS. On the Internet tab in AU, I connect via DHCP. A DHCP reservation is configured on the ASUS for the TC (and, well, for everything so that if I'm ever looking at a log or a packet trace I don't have to figure out what 192.168.1.72 - or whatever - is). On the Wireless tab, I create a wireless network, WPA2 protected. No guest network. Under wireless options, I give the 5GHz network a different name. All Automatic for channel selection. No hidden networks (a worthless "security" measure that causes more problems than solutions - don't get me going!).
This creates a "roaming" network for my 5GHz. Essentially, any number of wireless access points can advertise the same SSID and you'll see this in any large building that offers WiFi. A joining device will choose one usually based on signal strength. I'm about equidistant from the ASUS and TC right now and am connected to the TC (slightly better signal).
The TC is located on the opposite side of the house from the ASUS for WiFi reasons. Otherwise, I could put it anywhere.
In your case, the lack of wires means you'll have to create an extended wireless network as Bob indicates. You could wire the TC to Linksys (or its replacement) as I did. Leave the wireless off on the Linksys. Configure a DHCP reservation on the Linksys so the TC IP address doesn't keep changing. Create a WiFi network with the TC. Then you can use AEs in "extend a wireless network" fashion to cover the rest of the house.
You mention a Wii... Something I discovered a few years back is that if the Wii is in standby mode (yellow light), it repeatedly executes its network startup code - requesting a DHCP address, etc. - once every minute or two. It is a bunch of traffic on the WiFi that isn't going to kill things, but isn't productive, either. This doesn't happen when on (green light) or off (red light). So I've taught my kids to hold in the button until it turns red when they finish using it.