Implement feature not available in Parental Controls?

I would like to use (or write) a timer that I can setup to use with my kids on Mac OS X (Yosemite). The functionality which I would like, which I haven't found off the shelf is:

• Set the timer for a period, T (e.g. 20 mins)


• Print a warning to the screen at T-n (e.g 17 mins)


• Lock the computer at 20 mins


• Unlock with administrator access only

I've experimented with pmset schedule but it allows the user to override.

Parental controls as currently implemented don't work for this as we want to split their time into smaller units, and more than one kid uses the same login (and I've read that this feature is buggy under Yosemite)