There is so many other things that your kid is more likely to spend time online for that sex is overrated as a concern. Same with making bombs. It's much harder using the web to do that than AOL integrated services. If they have to give an address, create one for the family and have the mail filtered into one central box everyone can use. Reserve the personal e-mail for friends you know or school work.
Networking solutions are cheap and it is a lot easier than any user based filter.
You can also monitor their work assignments and website visits. You have to know how to use the machine. You cannot leave this in your kids hands. That is a bad idea, no, a really stupid idea. Computers are easy to learn and there are books the size and shape of coloring books which can teach you.