Use the 'don't talk to strangers' reasoning. They shouldn't ever disclose personal information such as their age, their school, even the city they live in. And, if they ever feel uncomfortable about anything they see or read on the Internet, they should not be afraid to leave and tell an adult about it. One easy way to keep track of what the kids are doing online is to keep your password to yourself. This way, when your kids want to go online, you'll know they're there.
Some parents like to keep the computer in a highly visible part of the house.
Maybe in the living room, where they can see the monitor from the kit-chen. Kids won't try to find objectionable stuff if they know you can see over their shoulder. If you're finishing up some paperwork, or working on your taxes, you can sit near the computer. It will not only let you keep an eye on the monitor, but I'm sure your kids will appreciate the company. But the best way is to keep them occupied online.