The act of kidnapping could constitute a criminal law breach or it could relate to family law and criminal law at the same time. Kidnapping is a crime that occurs when someone under the age of legal consent is held against their will and against the will of their guardian. Ultimately, the guardian has the final say as to whether or not a child has been kidnapped; for instance, parents can send children to rehabilitative boot camps against their will, where the child may feel like they have been kidnapped, but this does not legally qualify as kidnapping.
In any case where a stranger takes or abducts a child, they will be charged with kidnapping. A child's non-custodial parent can also be charged with kidnapping if they take and hold the child (going against the legal court order).
In all states, kidnapping is considered a felony offense, but the sentencing guidelines can be different between jurisdictions.