If there is no court order related to child custody, then yes, you can withold the child until the hearing without violating a court order.  That said, Family Code 3040 states that the judge should consider who will allow "frequent and continuting contact" when making a custody order in the best interest of the child(ren).  So in this case--you are saying the other parent is "withholding" but you are going to turn around and do the same thing.  This may not play well with the judge. You should really consult with a local family law attorney and give them all the facts before you do anything to hurt your case.Best of luck.