It's not possible to do that on your router with the asuswrt firmware*. The more powerful Asus routers like the RT-AC68U have AiProtection and Traffic Analyzer which will probably give you the information you want. Alternatively you could use something like OpenDNS Home or FamilyShield.
* You might be able to get website monitoring by installing a completely different firmware, like Tomato, DD-WRT or OpenWRT, but I don't know because I've not used them.
P.S. Be prepared for the inevitable replies from people with absolutely no knowledge of your situation telling you how you should be "parenting" your children (because of course they know better than you).