Who does a parent go to for help when the other parent violates a federal law?

I am divorced single parent of 12 year old daughter, and 11 year old severely autistic son. He has resided in the same state for the past 2 years. The children have been enrolled in school here and their pediatricians are here also. My ex-husband kidnapped them from me in another state on a supposedly trip to see them with me there. State is a home state and where all legal matters pertaining to them must be heard, according to the Interstate Uniform Child Custody and Enforcement &Jurisdiction Act. He now has them in another state and has just filed for temporary custody. How can he just take the kids and file custody according to this law and its regulations?