The given name of each kid is a reference to his/her code name. Numbuh 5 is Abigail Lincoln (the $5 bill shows Abraham Lincoln). Numbuh 4 is Wallabee Beetles (the Beatles had four members). Numbuh 3 is Kuki Sanban ("sanban" means "third number" in Japanese). Numbuh 2 is Hoagie P. Gilligan, Jr. (he is the second person in his family to have the name). Numbuh 1 is Nigel Uno ("uno" means "one" in Spanish).
According to the show's Tom Kenny originally was going to voice Numbuh One. Warburton later recast ed the role to Ben Diskin because he felt Kenny's voice take for the character sounded too old for him.