It is safe for your kids to use social media sites as long as you monitor their usage. There are parental controls you can set, permissions to grant and filters to apply within Windows and your Internet browser. If you open your Internet browser, go to the Help tab. Enter "Content Advisor" in the search box and you'll find the resource: "Help your children browse the Internet more safely." To make specific changes to control Internet use, there's a helpful link "Block or allow specific Web sites." Similare functionalities exist with other leading browsers.
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. Many Internet providers also provide parental control software so please check with your provider.
While these 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.