Esther would like to monitor her teenagers' activity online. Leo says that parents should have every tool they can, but one thing she can do is change her DNS settings to use OpenDNS.com. That's the "phone book" that routes web traffic to the appropriate addresses. OpenDNS has great parental filters and blocks, and a lot of schools and business use it.
Leo says that it's also possible to turn off the internet at night, either by unplugging the router or by scheduling it in the router's settings. The very best thing she can do is have that conversation with her kids about the Internet and what she expects of them. SafeKids.org has an Internet contract that will help lay out her expectations for their online behavior. Another thing she can do is give them a phone that allows her to track them wherever they are.