This question makes me super happy. Our kids are pretty into Beeminder -- blog.beeminder.com/faire -- but I'm kind of predicting they'll rebel as teenagers.
Anyway, does it possibly work to just add your own credit card for your kid's account? I'm actually not sure if we enforce that being a different credit card than the one you use for your own account.
If that doesn't work let's talk more about what we hack together to make something like this work somehow!