"....the ones that are interested in showing off or getting pretty dresses openly, are generally cross-dressers or drag-queens, because their desire is to flaunt a perceived gender transgression for various reasons (they think it's funny, they think it's fun, they love performing, they love freaking people out, it turns them on as well, etc.etc....
The ones who are mortified at anyone finding out, are sometimes fetishists (sometimes called transvestites), who generally are attracted to the sex whose clothes they're adopting and don't like people knowing because they think it's embarrassing, but could just as easily be transgendered and considered their "normal" clothes drag but don't want people to know because they're afraid of discrimination."
Hopefully that helps sort things out for you! Of course, every situation is different, and every person has their own individual reasons for cross-dressing, and their own personal preference for different lables...the best way to find out is simply to ask them.
In this story's case, I think the author is implying that the young boy has a transgendered identity and wishes to cross-dress because she feels more comfortable and more like "herself" when she does. The tomboy, on the other hand, just seems to be more in the middle of the gender divide rather than having a desire to actually cross-dress, tho' of course I've only read what bit you've read here and am totally guessing.