What's the best way to know what kids are doing on the computer?

I understand your concern. Children spend a lot of time at the computer. And they spend a lot of time on the Internet. Unfortunately, the content of the Internet is not always intended for children. So I installed mac keylogger https://www.refog.com/mac-keylogger.html on my kids' computer.

The software is designed perfectly for use on home computers. Parents can make sure their children are not going to illicit sites, downloading music illegally, or chatting with dangerous people. Parents can read chat conversations, check the screenshot history, and look at a list of visited web sites from their children's computer activity.

The main rule is not to leave children alone with the computer for a long time. So you can always see what games children are playing or what videos they are watching.