What should I do if my ex-husband hid assets and they were not considered when determining the child support?

This is a very serious issue. A judge would consider this to be fraudulent concealment of assets and would look harshly upon your ex-husband. Your ex-husband would have to pay additional child support, and he would be subject to additional penalties for committing a fraud upon the court. As you can see from the sample child support guidelines, fraudulent concealment is specifically mentioned when calculating child support. Your ex-husband would most likely be ordered by the court to pay back child support payments to equate to the new (honest) obligation.