What are the laws to be able to get them to come back to the US...if he decided to stay in Lebanon with the kids?

Irrespectively of what the laws are in Lebanon, and whether there are treaties between the two countries, international child abduction questions are very difficult and can take years to resolve. If there is any chance that he might stay in Lebanon with the kids, you should simply prevent him from taking the children abroad. You would need to ask the court for an order preventing child abduction.

You can see on the form at http://www.courtinfo.ca.gov/forms/fillable/fl341b.pdf what orders generally might be available to you.