If you are a parent, you can both add and deduct from the Piggybank. 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 Piggybank view. To access this, go to "Children" and click on the Piggybank for the child you wish to change.
If you are a child user, you can only deduct from the Piggybank. Go to the main view and access the Piggybank from there.