Rachel:
Ray, Cello,
Elle,
AceHannah: Han (like Han Solo) Nano,
Annie, Nani (NAH-nee)
Charlotte:
Charlie,
Lottie,
Lotta,
Cherry,
Art,
Chet, Cello
Jessica:
Jess,
Jessie,
Jessa, Jeca (?),
Jett,
Echo? Echa?
I am really just making these up as I go along. Just take random sounds from the name and distort them see if any appeal to you. At least, that's what I did.