That's just like Mommy and Daddy's own bank account sometimes, so we just have to wait for the bank to get more money in it. Allowances and salaries go hand in hand. If your child receives an allowance, you may be able to explain paychecks and salaries easily. Show your child what your paycheck looks like. This is the same as what we pay you as an allowance.
Daddy gets an allowance, too.
If you work hard and do all your chores at home, sometimes we give you a little more money. That's just like Daddy when he comes home a little later or goes to work on Saturday. He gets a little more money for working harder. Children can usually grasp the idea of working for an allowance if you do structure the payments like a regular job. Be consistent with the payment schedule, and have some incentives built into the system, such as bonuses for certain jobs or extra pay for doing a really good job on a chore.