Edit: HOLY CRAP! CHICAS CUPCAKE! baby lures kids with I've cream, chica with a cupcake, I bet.
I am convinced that chica caused a bite incident. "I was the first. I saw everything." She was the first bite incident, before the crying child's. And don't forget that she has her beak on the main menu and cutscenes, but not the gameplay itself. Also, when she says, "there's only one thing that could go wrong" in the cutscenes, she has her finger pointing on her beak. She had an image in the Logbook of herself hunched over. She isn't in sister location at all, but her name is mentioned in the source code leading up to it's release, mentioning a "Chicas Party World." This likely means that another company had owned chica before merging with Fazbear Entertainment, and I suspect that this is where the bite happened. Another note is that part of Nightmare Chicas mechanic is jaws closing down on you from the vertical sides of the screen.
You won't get tired of dying, will you?
Where's my beak? Lodged in your forehead, of course.
Let's go somewhere more private... So I can eat you.
That's right! And don't you come back now, ya hear?! (This reminds me of the Juniors guy's line in fnaf 6 midnight motorist game)
I may not like wet floors, but the smell of fresh meat is just too enticing!
Whoops, looks like you're the one that slipped up this time.
Let me show you how to break your face and look like me!
I have seen him. The one you shouldn't have killed.