my daughter is 6. no TV in her room (at either parent's residence). whatever she wants to watch, i watch with her. that way, i know for certain how much time she's spending, if it's appropriate for her age, and i'm there to answer any questions.
she wants her own PC, and I'm going to build her one, but it'll be on a KVM switch with the main house PC, again, so I know what she's doing, how long she's doing it, etc.
kids are always testing you, seeing what they can get away with. rather than come up with a high-tech killswitch to control her screen time, i prefer to stay involved. As a result, I can only watch my shows or use my PC when she's not around, but that's the sacrifice I've chosen and accepted.