@aasskkeerr,
I'd pay them nothing and then call the cops 'cause I have no kids. They'd be arrested for B&E if they were in my place.
Seriously, I would only give an allowance if they did chores around the house: lawn maintenance, taking care of trash, washing windows, washing the car.
I'd pay whatever the going rate was in the neighborhood for those chores. About $10/hr.
No chores done? Then there'd be no allowance. There's no such thing as a free lunch - even at age 12.