Pantsless_Bob and other kindergarten aunties probably don't remember themselves as being cruel. I have some examples. Warning! Do not read if you have emotions. The reader has been warned.
I and my friends dissected a mouse with a paring knife when the mouse still was still living. It was fun to watch when a cat ate its brain. I also put live bugs into turpentine, and watched them die. We burned some ants with gunpowder in a small paper boat. My friend prevented his friend T. from going into the lavatory, so T. shat in his pants.
So don't tell me that children wouldn't be cruel. They are sadists, and have no empathy. The "immediate desire" that P_B refers to can also be to see something or -one suffer. Children are also masters in contempt: bullying is usually thought as a school-specific problem. They're capable of malice that adults see as completely inappropriate. They haven't learned about suffering, so they think that it's fun to watch. The concept of suffering other than the of themselves comes later in life. Until then, they don't kill each other only because Mum tells them not to. "Cool, you can put a bullet into his head!" When you have a bunch of boys together, the result is, if unsupervised, as in Lord of the Flies.
The life of a child is usually thought to be "care-free" etc. The hierarchy of a group of children is much harsher than the one of adults, as the example of T. illustrates. When I was a child, I was really annoyed by the idea that children would be always happy and having fun all the time. I was treated like a retard by the adults just because of my age. I hammered it to my brain that children are cruel, that they oppress the "unpopular" harshly, and the adults think they're stupid. Children know less and understand less, but this does not mean that they are stupid and happy.