|I don't think you are parenting.|
If your daughter and you disagree on some topic or agreement, you should use reasonable means to correct the issue.
Simply unplugging it or "Is there a way I can stop her accessing the router without her knowing that I know.?" is rather childish.
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10