Legal Question in Family Law in California

It is an agreement but not enforceable by the court unless it is a court order.

You need to be especially careful if there is an existing support order and she agrees to take less, because if she changes her mind and you haven't turned the modification into an order, you will be responsible for back support with interest.

The law is clear that parents cannot waive the obligation of child support between the parties. Therefore these agreements tend to be unenforceable in court.