It all depends upon the bird. Like people, they all have different personalities. Your DD is old enough to understand (I know it's hard, but kids are resilient) that if either or both of them turn mean, they must go. It's a matter of safety for her and your little one. I wouldn't worry about it too much right now - just be prepared. We live on a farm, and my boys took it much better than I did whenever we lost or had to butcher an animal. Just start getting her used to the idea now. I'm guessing that eating either of them wouldn't be an option, so you need to tell her that they will be going somewhere else. Tell her now, and tell her often. Over the years, I've had to teach myself not to get too attached to the animals. It's easier that way when something happens to one of them.
As to your question about "half staying sweet and half turning mean" I don't know if it's quite that simple. The ones you see in pictures are obviously ones that have turned out nice (or are being handled by experienced kids who have grown up with chickens and know how to handle a mean one).
