Steam has some of this built in

It won't completely stop them from playing, but you can moderate a bit.

When I was that age, I used to hog the phone line dialing bulletin boards. That was easy to control because my mom would just pick up the phone and mash the keypad until I got disconnected. Today, as much as I hate to say it, controlling network access is a more reliable way. Your home router probably has a guest network, or if you're really sneaky you can block steam IP addresses when they have to get homework done.

It's hard to say what's an "appropriate" level of screen time, especially now when their options for entertainment are limited. I'd use hard capping as a last resort when encouragement fails.

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