I wonder if maybe he’s just writing down his kid’s quote word for word and putting it in the comic, as a sort of ‘Kids Say The Darnedest Things’ thing?
Still weirdly specific and out of nowhere.
ETA: Oh, I think I see what the joke is supposed to be. Mom is dragging out all these old, simple, classic fabric costumes for the kids, and Billy suddenly decides he wants to go from the easiest costume ever to something not only highly specific, but strange and difficult/expensive to do, presumably right on Halloween when that kind of thing can’t be done short-term.
Still weird.