As far as the names made a few suggestions way back when -
https://community.cbr.com/showthread...=1#post3869352
https://community.cbr.com/showthread...=1#post3567255
If Freddy is King (besides the Kingdom Come Elvis nod) they could all go by Chess designations: King, Queen, Bishop Knight, Rook.
Although they are not using Marvel, there is no reason they can't still use Captain, and by extension expand on this with the others whimsically all taking Military rank designations, Billy obviously still be Captain, the others different ranks; Lieutenant, Major, General, Admiral, etc... I can see kids having fun with this.
Combine the Rank with their respective power attributes: Power, Courage, Wisdom, Strength, Stamina, Speed.
Another suggestion was just easily referencing their respective Deity Namesakes: Hercules, Zeus, Mercury, etc..
Or even further (to the horror of some here) fully embrace the Power-Rangers motif using their color designations: Red, Green, Blue, ... etc. Or again the rank or attribute combined wit their color .
All fun!
Also Johns so far is having great fun with it: and the kids debating what name to go by, pretty much their own discussions mirror several ones we had here.
Although I can't remember anyone suggesting Captain Sparkles! Or the Sparkly Friends ! LOL