Daidaiboshi - Manga
Summary
From Aerandria Scans:
Long, long ago four witches of great power were sealed away in a mysterious kingdom ruled by a mysterious king. To this day, the legends say that the one who finds their graves will have their wish granted - however impossible that wish may be. Daidai, a girl who escaped from the king's treasury, believes in those legends and starts her journey through the magic kingdom, accompanied by Matsuba, a small boy-robot who loves her. What really is their destination? And what wish shall the graves of the four witches grant the unlikely companions?












In a system like that there would need to be rigorous documentation and certificates of prior ownership/guardianship or else anyone and everyone would just get grabbed off the street and enslaved at any time, and such a society would collapse instantly. So these slavers wouldn't have such documentation unless it was a forgery since the kids were just kidnapped; both offenses should be very serious capitol crimes. Once again, the day is saved by glorious Bureaucracy! (and the MC's knowledge and exploitation there of).