Each underworld is different for each mythology. Even Gods don't come down there freely. Keep the myths separate and the high Gods out. Campers must have permission to leave camp before heading down and then they can't just come in, they have to journey in and not get caught. Remember getting out will be a lot harder.

Greek:
Hades, Persephone
[Will allow Demeter occasionally]

Roman:
I kind of skipped over this cause Pluto doesn't have children.

Hawaiian:
It's not the underworld, it's Hunamkoku, an island in the clouds where good souls go to after death. The bad souls go tot he land of the dead.
Hunamkoku: Kane [Will allow Lono and Ku]
Land of the Dead: Milu

Japanese:
I'm not really sure so I'll just allow the fortune seven; Benten, Bishamon, Daikoku, Ebisu, Fukurokuju, Jurojin, Hotei

Celtic:
Arwan, Macha, Sucellus

Norse:
Must have missed this one...

African:
They have more of a heaven and hell sort of set up. Gamab directs the souls and from there demons and spirits do the rest.

Aborigine:
Kultaba

Voodou:
Baron-Samedi, Maman-Brigitte

Inuit:
Sedna, Tapasuma, Anguta