We use Khan Academy and Ron Paul Curriculum, and pay them one Sat per second they are productive at learning.

I require them to calculate an invoice and bill me for the time and include proof of work (written or online energy points). When they want to buy things I allow them to withdraw from their wallet in exchange for cash by sending them an invoice, or we look at receipts at home and level up in the evening. They also get paid for chores but only if they actually do them and its fully on a gig basis, they can compete to do more of the chores.

This notion of invoicing me for work they do shouldn't be a foreign concept to them because in the real world you don't get paid in business unless you bill your customer. Its important to experience that if you don't bill your customer right away, you will probably never get paid. This is because the value of services rendered decay with time and the proof of your work becomes much weaker evidence as time progresses.

It may sound strange to use money in the household, but I'm against the principal of an allowance for existing. Its my responsibility to ensure they are fed, housed and clothed, but if they want something that's not necessary and its not for everyone else it's up to them to save up and pay for it. That's not to say they don't get plenty of gifts besides all of that.