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).
Each season in the series features one or more "art shows". They are experiments in various artistic styles and thus differ significantly from typical episodes. Some examples of these would be the first season's Operation: T.H.E.-F.L.Y. (an episode where no dialogue is spoken and the story is told through music), the second season's Operation: R.E.P.O.R.T. (each of the five main characters gives their version of the events of a mission in five different animation styles), the fourth season's Operation: F.O.O.D.F.I.T.E. (a heavy metal rock opera), and the sixth season's Operation: S.C.I.E.N.C.E. (a presentation of commercials that, in the episode's storyline, characters have created to advertise their 2x4 technology).
The Kids Next Door were originally meant to be the neighbors (hence their name) of Kenny and Chimpy, who appear in "Diseasy Does It", Tom Warburton's What a Cartoon Show short. While Kenny and Chimpy did not become a series, the Kids Next Door did and retained their name. A character from the original show "Professor XXXL" (Triple-Extra Large) was made a villain of the newer series.
In "Operation: A.R.C.H.I.V.E.," Father's real name is revealed to be Mr. Wigglestein. However, later episodes (especially Operation: Z.E.R.O.) contradict this, perhaps demonstrating that Numbuh 1's flight of fancy was exactly that.
According to the show's Tom Kenny originally was going to voice Numbuh One. Warburton later recasted the role to Ben Diskin because he felt Kenny's voice take for the character sounded too old for him.