How do I keep my husband from taking kids out of state?

You really should have legal representation on this; much is at stake here.  If money is an issue, see if you qualify for Legal Aid or see if they can recommend someone to help you.  Also, check if there is a law school nearby to where you live; they typically run free/low cost clinics that handle these type cases.  Contact the local Bar Association in your county; they may have a list of attorneys who will take your case "pro bono" (for free) or at least for a reduced fee based on your income/circumstances.  Additionally, google women's support groups in your area; they can usually provide a list of attorneys who can help.  Finally, contact your state's Department of Social Services, they may be able to refer you to free legal services.

If there is no court order in effect, than he can take your children out-of-state.  However, you hcan go to court and obtain an order which will require him to remain to the state.  If at that point he leaves,  he can be charged with parental kidnapping.

The court will consider the bests interests of the children in making its determination. The court will look at the age of your child, the relationship of the parents and the child, the present living situation of the child, etc. 

Note:  Before an order is established in the state you are all in now, he could choose to leave with the kids and file for custody once he re-establishes residency in his new state. So be sure to get to court with an attorney ASAP.