Daughter in-law is leaving Germany with sons children who are US Citizens wBrazilian Pports can she leave?

There was no discussion of leaving until today and he needs to know can he detain travel until some Affadavit of Authorization has been established? I told him to contact the State Dept. At least put a flag on the pports. They are his legal children, but she is abandoning him and leaving Germany with kids to live in Brazil.