You probably don't want to make that argument. If your drawing is simplistic/symbolic enough that people have to rely on "tertiary sexual characteristics" like that, you probably need to work on your art more, or revise the character design. Unless the character is supposed to be a bit androgynous. Which, to a certain extent, is true for small children--there's relatively little difference in terms of body or even face between a six year old boy and a six year old girl. There is some, but it's muted, and much of how we tell the difference between the genders of children that age is tertiary stuff like clothing or how they carry themselves or how they talk.
Really pay attention to things like brow ridge, chin, and jawline. Lips, nose, and eyelashes are important too, but a lot more variable, and tend to be the things that are quickest to get turned into symbols when drawing. Jawline is especially important.
For the body, if you want a character to be emphatically female despite a butch hairstyle, make sure you go with lots of curves if the character is female. I'm not necessarily talking about boobs or even hips, so the fact that it's a "little girl" isn't an issue. Always using a curved "line of action" when posing the character can be a help, and making sure you know the difference between how men stand and how women stand, for example. Understanding the skeletal differences between men and women can be vital. A slightly pear-shaped body can help, but isn't a perfect solution since men can easily be pear-shaped, too, especially in cartoons.