(d) Ensure that migrant children are promptly identified at places of first arrival in countries of transit and destination, and, if unaccompanied or separated, are swiftly referred to child protection authorities and other relevant services as well as appointed a competent and impartial legal guardian, that family unity is protected, and that anyone legitimately claiming to be a child is treated as such unless otherwise determined through a multidisciplinary, independent and child-sensitive age assessment;