Patty and Marci voiced by boys?

Boys are voiced by women all the time. It makes sense, because their voices are still high-pitched.

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Charles often played with 'misconceptions', like 'The Great Pumpkin', which was just Linus confusing christmas and religion with Halloween (originally).

Kids don't know the world as well as adults, so they make all kinds of mistakes until they learn how things really are. I would be perfectly within reason for a kid to not realize how genderized 'sir' is supposed to be.

Also, Peppermint Patty is a tomboy, without feminine qualities that would signify her as female, the way many others do. Even Lucy is more 'clearly female', despite being masculine in some behaviour. Patty looks, sounds, and behaves like a boy, so obviously 'sir' would fit her in Marcie's thinking. (Drat, I made the mistake of calling her 'Marcy' previously, because someone else made that mistake and I trusted them instead of researching..)

I think Marcie calls her 'Ma'am' in one of the comics.