My ex husband living in CA with my kids I'm living in Vegas. He suing me child support. What should I do?

Consult with a qualified family law specialist.  The case might (or might not) be proper to have brought there; if not, you could and perhaps should be in court here.  Either way, you should know the rules relating to jurisdiction (see "The Basics of Family Law Jurisdiction," posted at http://willicklawgroup.com/published-works/) and the applicable child support laws.