they check children's birth certificates only at port canaveral. i think the idea is that if they are cleared at the port, they are okay. adult documentation is what is scrutinized.
i spoke with my rep at dcl and she said that ins (not dcl) requires the raised seal. copies are not acceptable. she has a uk-born, but american citizen, son with a special certificate that states this. when she has gone on the cruise with her family, dcl at the port always takes this document to someone backstage to ensure that it is acceptable proof of citizenship, so there is an ins rep somewhere backstage in case of questions.