You can make them a limited user account and enforce parental controls (both in the OS and maybe on the router) if you really want to. Don't be surprised if they find workarounds...
Back in my day, my peers and I just had unfettered root (well, admin) access to our machines. I dunno if it's realistic to protect kids from all the things anyway. Whatever you shield them from is just going to seem even more appealing, and no doubt their friends will find it for them if they really want it.
