Sounds like replacing the router was an exercise in frivolousness, because the trouble began when your kid downloaded something. Disconnect the kids’ computer, figure out what he downloaded.
And if that computer uses XP or Vista, ensure that each kid’s account is configured as a basic user, not an administrator. They clearly have a problem regarding program installation.
We did disconnect the kid computer ~ still no luck.
The kid downloaded lots of crap, we are in the process of uninstalling all of it.
Admin rights will no longer be available on the kid computer.