Better would be something along the lines of "You were only so good this year, and the time is up. If you want to talk more, you need to earn more good points with your mom and dad!"

No idea how you'd monetize that, though.

With 'in-app' purchases of course: 100 brownie points now only $10, hurry this offer won't last.

Somehow this device fits well with the Don't be a sucker video linked to elsewhere on this here site [1]. Good advice, valid in many contexts. Don't.

[1] https://news.ycombinator.com/item?id=45573025

Nah - I want something that one can monetize and actually makes the kids be good (somehow).

Perhaps a parent commitment that if the kids earn X many goodie (goody?) points, then the CC is charged, and let the parent control how they earn those X points.

Gamifying good behavior has been shown to be pretty effective with kids. See Kadzin.