Can I revoke a court order for a child support, since the I never been served and...?

The children are illegal aliens. I'm legal resident and my ex wife and my kids are not but Child support office contact me after 6 years of an order that she put on 11/2004 and she never mention it. I give money to my kids(now 22 and 20) but always was cash or western Union. And I have my son live with me mostly all the time.But I never been served by the court or getting any certified letter from them.