Portuguese to English Law/Patents Law (general) documents

this is the title... Here in Brazil it is very common to issue this document for minors who are going to travel (by bus, train or plane) between cities or states or even internationally. Both parents must sign it and, ideally, one would not be able to purchase a ticket or board a bus, train or plane without it... This is an attemp to prevent children and adolescents from running away from home or being taken...
any suggestions appreciated
tks