If you are a parent, you can both add and deduct from the Demo account. If your child has bought something, or you have paid out the allowance in cash you make a deduction of the corresponding amount. You can do this by clicking the plus or minus sign in the Demo account view. To access this, go to "Children" and click on the Demo account for the child you wish to change.
If you are a child user, you can only deduct from the Demo account. Go to the main view and access the Demo account from there.