Children who are cruel to animals: when to worry.

I don't think it's uncommon for children to exercise their power over smaller and weaker creatures. The article acknowledges this as "The Experimenter: (ages 1-6 or developmentally delayed)."

 
Kids will do stupid things and often don't realize the potential consequences of their actions until it's too late. I think it's important to distinguish between the ones who momentarily have a lapse of concern for the animal and those who habitually abuse animals. The ones who have no sense of remorse and repeat the behavior are the ones who might grow up maladjusted. As the article points out, this is why it's so important for adults to cultivate in young children a sense of empathy and respect for all living things.