There’s software out there to audit and keep track of who goes where. My cousin bought a program that when you sign on (userid/password) you have a limited amount of internet usage, it basically counts down your time, I.E. 2 hours a week. Also, make it clear for your kids what you consider allowable and the reasons behind it and as with any parenting, give them the consequences of not following the rules.
Take the privilege away if need be, I’ve done this only once and my kids and they respect their time on the computer. Age appropriate websites, you’re the parent, you set the boundaries it really is simple.