I think its just a kid being a kid. At that age they tend to stare at anyone...who knows what thoughts are going through their brains. Now if the kid said something like, "You look like a boy", or similar to you, or heard questioning something of the like to her mother regarding you, then you got clocked. Most of my troubles were from small children, they see everything in minute detail, noticing things that most adults miss, from adam's apple to subtleties in your voice or mannerism, or your height or body shape, and many times they can't help but vocalize what they've discovered (yeah, announce it to the whole world why don't you). As unnerving as it is, I think it was just a staring child, some stare, some don't.