Ugh - as a long time animal owner and the mother of 5 (boys and girls), I feel the need to point something out, folks:
NO child truly has reached enough of an understanding of right and wrong to be trusted to make decisions on their own until *about* 8 years old. This is fact, not my personal opinion. My personal opinion isn't that lenient! ;-)
Seriously, there is probably nothing "wrong" with the child who tossed the chick; 6-year-olds are not capable of the complex reasoning and caution that leads to common sense yet. They are still learning how to behave and respond to the world. Several people already pointed out how it may have been a simple, internal, "will it fly yet?" kind of impulse. I have heard so many parents say that their toddler or young elementary aged child would never do {insert behavior here}, but the reality is that they WILL unless they have learned never to do that one particular thing AND have the impulse control for that particular rule.
It's not an issue of wrong or right, it's a combination of age, emotional/intellectual/social maturity, and experience. Any child development professional will tell you the same thing. Point is, do NOT try to place your adult understanding and standards on a 6-year-old child. Every single one of my kids has been raised around animals, birds, and reptiles of various types all their lives. Only one of them seemed to "get it" by 7 or 8 years old and they are all highly intelligent, non-ADD or anything else, kids. My 4-year-old is in love with all of nature, but she's still not capable of truly grasping the difference between dolls, toys, and live animals. She got a new play-dress yesterday and promptly told the dogs all about it. She'll even tell you they can't talk and all that, but that's not the important piece.
Back when raising Chihuahuas, I had parents come over wanting a puppy for their child. Only once did I see a child that was got it well enough that I felt safe letting a dog go home with them. The others were more or less perfectly normal kids who loved dogs, but I didn't trust their parents understood the dangers enough to constantly supervise.
Remember the Looney Tunes character, "I'm gonna love him and hug him and name him George!"? It would be far safer if every parent operated as if that was the expected behavior of their child - at the very least, it would help many of us to not overreact when something bad DOES happen.
I hope your daughter is better and isn't too upset by the whole episode itself. If that chick does somehow show evidence of damage, she may feel guilty and see herself as being the reason. HUGS! Chicky sounds like it's fairly resilient.
PS: And yes, there are some really messed up kids out there who would do something like that to act out all kinds of issues, but let's try to be optimistic; these are her daughter's friends!