This is brilliant. DNS + API is a really smart way to build this. If you haven't done it already, it would be pretty straightforward to post a notification to the child's device (via a companion app on selected platforms) as an additional cue to let them know that they'd used up their time.
I'm halfway tempted to set this up for _myself_. You're absolutely right that 30 minutes is gone in instant when you start in on online entertainment.