Rules around computer use for pre-teens?

[MrsFogi]

Hi could anyone share what rules they have for their dc's use of computers/screen time? I am feeling a gradual creep of screen time increasing and computers being used in bedrooms behind closed doors. At the moment it's DanTDM on a loop so I'm not concerned but I'd like to have a framework of rules in place and followed now before it becomes impossible to enforce and before the dcs get more "online" and interactive with the outside world. Could anyone share their "rules" that work for pre-teens and (ideally) teens?