[hrktb]

We have strongly enforced limited time (the device itself lock when allowed time is over) and softly enforced ("unlimited" until some deadline).

For us it helped a lot to have reminders firing off 10 or 15 minutes before any hard deadlines (e.g going to school) and warning them 5 or 10 mins before dinner or going somewhere.

They're still not happy, but they take it way better and learn to do whatever they really want to do in that remaining time.