i would like to add a second network just for the kids to my xfinity router do i just attach another router via ethernet cable to my current router
That's one way to go about it. Most of the devices will let you broadcast 2 SSID's one for each 2.4 and 5 ghz bands. Can they get a signal from your modem in the room they're trying to connect from? Are you looking to segregate the traffic or something else?
Depending on which router you have, you may have several options. First would be to set up one channel for the kids and the other for everyone else. Note that the 2.4 has more range but is slower. The 5 ghz is faster not does not have the range. Also some older devices may not work on the 5 ghz. Some modems have “guest” options where you can set up additional networks so you can have four separate SSIDs. That would be the preferred option. Google “guest network router wireless” and you will see most router companies have them. I also recommend that you buy from a company that has a return policy if it does not work for you.
There are ways to add a router behind another router but takes a lot of setting up knowhow and some routers do not play well together. Hope this helps.
The Comcast supplied gateway devices do not have a guest network feature. Comcast instructs users to have their guests use the public WiFi hotspot broadcast SSID from the gateway as a guest network if they wish;