This is not a complete answer, but I couldn't fit all of that in a comment. I also don't have any expertise in neither biology nor anthropology, so you should probably take this answer with a grain of salt.
As many others here, I'm not sure that adolescent animals are really less violent than children. I used to keep rabbits and although young bunnies tend to be less aggressive, sometimes there are exceptions. If not stopped, they can mutilate each other pretty horribly.
Still, there are lot of differences between humans and other animals that affect both human behavior and your perception of it.
Compared to other animals, human growth to adulthood is very slow. Most other mammals progress from the short period of infant dependency (toddlerdhood) directly to juvenescence. (1). Humans have 10 years of childhood. Children are not yet fully developed, but they are already fully operational: can walk, fight and use tools. This stage is long and unusual. Other animals usually don't have a lot of time to play and be violent to each other.
Children have a lot of time to play and to fight with each other. Wild animals don't have this luxury, they have to hide from predators and provide for themselves. Less time in general means less time for play and fighting.
Children are not trained to withstand pain and tend to cry often and they do it loudly. Crying with tears coming out of the eyes is unique to humans and children cry in response to minor physical and emotional discomfort (minor falls, verbal insults). Crying child is likely to get noticed by everybody in its vicinity.
Wild animals have their own reasons to not get noticed and have to keep quite all the time, even after severe injuries. Even domestic animals have a higher sensitivity threshold and are less likely to vocalize in response to pain. Violent games between animals are less likely to be perceived as such because animals tend not to react "negatively" to them. Kittens playfully scratch and bite each other. Calves head butt one another with no mercy during play.
Bear cub is expected to play with at most a couple of siblings during childhood. Young wolf grows in a pack of 5-15 other wolves, most of them adults. Herd animals live in bigger groups, but they are mostly herbivores and less likely to hurt each other during play.
Children, on the other hand, are routinely locked in small areas with hundreds and thousands other children with little to no adult supervision. That gives a lot of opportunities to pick up a fight.
To make it worse, children have human intelligence and their social interactions are extremely complex compared to juvenile animals. This provides a lot of reasons to fight and to hate each other. Lion cubs can't speak and, thus, can't insult each others intelligence and physical shape. Young dolphins are unlikely to mock each others heritage, social background, religious beliefs, political affiliation or sexual preferences. Wombat joey probably won't steal other joey's bike, etc.
The previous paragraph applies equally well to adult humans. Human interactions are uniquely complex. Animals, young and grown, can be violent to each other, but our concepts of hatred and cruelty can't be easily applied to other animals.