If you astronomers do not know what Pluto's family are named after,I will tell you:
Pluto is name after the Roman God of Underworld(The world where dead living things' sprits(or souls) are living),as 11 year old girl Venetia suggested,as Pluto is very far away from the Sun,it is probably a very dark and cold world out there.
Charon is named after the Greek ferryman of the dead sprits to ferry them to the underworld.
Hydra is named after a Roman mythlogic 9 headed ferocious creature who is said to have a link to the underworld.
Nix is named after the Greek Goddess of Darkness and Night,which is also similarly suits the moon like Pluto as mentioned above.