Honestly, "Secret Keeper Girl" fails right off the bat for using the word "tweens."
I don't like the word "tweens." Frankly, the word "pre-teen" means the same thing, is less confusing (when I first saw the word "tween," I honestly thought it meant "people in their early 20's"), and doesn't sound really, really weird.
However, I will say that the "panties test" aspect is right on. You should not be wearing pants so tight that we can all see the exact shape and/or color of your underwear. Unfortunately, I usually have to settle for pants that are either too big in the waist, too tight in the butt, or too damn long. I actually have to wear control-top pants to work just so they aren't hanging off my waist.