haha I wouldn't worry too much about folks trolling you around these parts, seems pretty quiet from what I've seen of it.
I guess a big question is, do you typically like girls (or for that matter boys) your own age? If so, I wouldn't worry too much about this. It gets more complicated if you're more into children than adults.
Otherwise, I'd say this is probably a pretty common occurrence, more common than people like to think. Unless you start to feel like you'll have trouble controlling yourself, I'd advise you just to enjoy your time with this girl and not worry too much about it. Crushes are hard to deal with generally, and its compounded when you have one for a kid. Just try and be friends and have fun
As to why this happens... nobody's quite sure. It's not clear that there's even a *reason* per se, or that the question is particularly meaningful in the first place. It just kinda... happens. The problem is that most people aren't aware that it can happen to 'normal' people, and child care providers are generally unwilling to address it right off the bat - that when you're working with kids, sometimes you develop feelings for them. And I know personally how scary it is, but try not to worry about it and enjoy the time you have together :p