Either parent can take children on any trip they wish without the consent of the other parent as long as the parent intends to return. Parents have the right to know where their children are, however. Law enforcement and the courts consider refusal by one parent to tell the other parent where they are going and when they will come back, and contact information as evidence the parent is attempting to withhold the children from the other parent; that is illegal in most states ( parental withholding)
Even as married parents, one parent can file an "ex parte emergency Motion for Temporary Custody" to prevent the travel plans or have the child returned from out of state. "Ex parte" means you testify to the judge by yourself without waiting for a hearing that the other parent can attend. Tell the clerk and the judge that your motion is “ex parte” because of the emergency of the other parent taking your child. If the judge gives you an order of temporary custody, the police will try to find the other parent and serve him with a copy of the order. The other parent will have to comply or face contempt of court. You will then have to go to another hearing, later on. At the hearing, the court will decide what to do next about custody.