And, in virtually every state you can file a motion to dismiss another party's custody petition. Going directly to prosecutors, police, schools and welfare departments where you know or suspect your child was taken may well not work. The main reason is that there is no law directly telling such authorities what they must do. However, in many cases it should be tried. The reason is that laws and practices in this area are changing.
Some states do require the police, etc., to give some cooperation.
In other states and communities, no law requires it, but the police and schools are very helpful anyway. And often this helpful attitude extends to going out and picking up the child, and holding that child till the arrival of a parent with a valid custody order. Filing a missing-persons report with local police is an important first step. There are parents who have taken a certified copy of the valid custody order, gone to another community and picked up the child, and lived happily ever after. However, lawyers advise against it nowadays because of all the known and unknown dangers that complicate these cases.