To prove she's an American citizen to get into the country. If you arrive by all, passports are required by everyone.
If you arrive by AIR - not all. :(
DL would have been able to travel on her mother's US passport. Since fatherhood is up in the air - Dannielyn is temporarily stranded in the Bahamas.
Virgie might be able to apply for a separate US passport for Dannielyn at the US consulate/embassy - she would have to have documentation as the legal guardian.