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.