When you link a bank account, the Piggybank is replaced by the bank account you linked to your child. If you still want to use the piggy bank in parallel with the bank account, you can go to "Settings" -> "My children" -> (the child you want to manage) -> "Show Piggybank".
Note that the weekly/monthly money is paid out to the bank account. Any bonus is based on the amount in the bank account and not the Piggybank.