Chapter 1: Section 4: Page 27: Line 34 (406)

 The child, meanwhile, having caught sight of the Chums in their summer uniforms, stood gazing, her eyes wide, as if deciding how well behaved she ought to be.

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Can a four or five year old decide how well behaved they're going to be? Isn't that like expecting a cat to behave properly when guests drop by and not to wave their pink butthole in the guest's faces? Aren't five year olds simply themselves with no real thought or consideration? This Dally must be pretty mature for her age. The definition of maturity is being able to choose not to be an annoying asshole. Children often can't make this choice; adults nearly as often can't either.

"Chums in their summer uniforms"
Remember, the Chums basically look like Stripesy from DC Comics:

But my assumption is that they're wearing short pants. And the fitted shirt is actually a blazer. And they probably wear their underwear under their short trousers.