Once they're on an approved site, it's up to you to teach them to follow the guidelines recommended and to adhere to your parenting rules. In addition to checking their privacy settings, teach your kids not to "click" to install anything, ever (especially when it says you need it to watch the funny cat video). It's a trap for downloading "bad" software that will infect your computer. You should also set the PC to routinely run current antivirus and antispyware software.
Parenting magazine has published an excellent round up of Social Networking Sites for Kids, The most popular digital communities for every age by Melissa Rayworth. Most sites allow kids to only post messages using prewritten, generic phrases or words from a tightly restricted dictionary. Kids can't share personal information like a home address. With younger children, experts suggest joining the site with your child and friending one another so you have visibility into their activity. It's also a good idea to have a look at the online activity of your child's friends.
An additional resource for you as a parent with concerns about your children's online safety is Wired Safety.
While the following tools help to keep your child safe, it is ultimately up to you, as the parent, to keep your child safe by directly monitoring what your child is doing on the Internet. No web controls can take the place of active parental involvement and oversight.