I use Parental Controls with my grandkids and it works well. The problem is that when they walk away from the Mac the clock keeps ticking on their time limit. Is there some way for them to stop the clock?
Yes. When you've configured a time limit based on the amount of time that the Mac can be used over the course of a day (from half-an-hour to eight hours) the clock can be stopped by logging out of the controlled account. So, simply instruct the grandkids to choose Log Out from the Apple menu when they want to go off and do something else. The Mac will switch to the login screen and the ticking timer stops.
When they next log in, the timer starts again from where it left off. (The kids can see how much time remains by clicking on the menu bar clock.)
If you'd like the kids to start using the Mac again without begging you to log into it, just give them the password for that account and they can log in themselves. Before giving them that password, triple check that you haven't created a controlled account with administrator's privileges. Without those privileges they won't be able to engage in much mischief.