The he author include the detail about the Tomkeys' "DON'T BE GREEDY" sign in Paragraph 11 by D. It helps to justify the neighborhood's view of the Tomkeys.
The author includes the detail about the Tomkeys' "DON'T BE GREEDY" sign to highlight their perceived out-of-touchness with social norms. The sign is a stark contrast to the typical Halloween tradition of trick-or-treating, where children are encouraged to take as much candy as they can carry.
The Tomkeys' sign seems to discourage this practice, which further reinforces the neighborhood's view of them as strange and unconventional.