This will be a learning experience for you and your family. No matter what you're responsible for what is in your bag(s) and for the declaration made.

I too thought CBP 591 was a complaint form? Do you feel the officer made a mistake in fining you for the violation? Because sorry to say you're wrong then. If it's a different form please let us know.

I have Global Entry and declare when i am over my allowances and almost every CBP agent laughed that I ever declared it but I'm following the rules so that I don't get "caught" and have my membership revoked.