Limiting daily usage of web videos?

Hello,

This is likely a wrong forum to ask this one but anyway bast what i know.

I trying to find way to limit my kids "video" time in day. They mostly 
use YouTube from their tablet's and computers.
Does someone have idea how i can monitor time spent on site per host? 
It's pretty simple to drop incoming packed when time is exceeded.
There is time based rules but what i want is to give example 1h token 
every day what kid can spend on many pieces.


Only idea what i have is to write script what coutting traffing to/from 
youtube every second and control packed based this information.

All ideas are valued :-)

Tommi