Kidnapping is the act of stealing a person away without their consent and/or without the consent of the people in charge of them. It is both a Real World crime and a PPC charge. As such, it is highly recommended that you do not kidnap anyone, as it is a very bad idea.
While actual children are possibly easier to kidnap (though not necessarily easier to deal with after taking them), many fanfic writers instead set their sights on canon characters. When an author steals a canon character away from their home continuum to the Real World (often in order to have a cutesy sleepover or some such), it is called Trans-Dimensional Snatching and is handled by the Despatch department.