You can't, but a better solution is to setup a new, managed account for them with parental controls turned on. Parental controls has a feature where you can review logs that show sites they have visited and they cannot access that log.
This first link shows you how to setup the account and the second discusses the logging.
Weaselboy's advice is spot on. Create an account for your kids, individual accounts would even be better, and use the parental control settings for these accounts. You will have a great deal of control and view into all of those accounts.