I’m no child psychologist, but I am what you might call an expert on assholes, so I’ve done some extensive research in the field of tattling. A couple key points to consider:
Basically a lot of the literature out there asserts that tattling is a way for children to receive attention. Attention-craving children will try to receive attention any way they can, even at the risk of doing something that they may otherwise know to be wrong. Furthermore, focusing attention on the tattler only encourages this behavior. By giving into the demand for attention that the tattler is looking for, you may give them the idea that this is a real way to become fulfilled.
It can also be about a grab for power as well; a means to get revenge on another child who they feel has wronged them in some way. “It is a quest for power, and tattling fills that inner need. These children long to see another child punished or perhaps delight in how they have managed to control the adult’s reaction,” says Jennifer White, some author I just looked up five seconds ago who writes about this sort of thing.
“At other times, tattling reflects children’s self esteem issues,” she continues. “Children may lack the confidence to handle the situation independently. Further, by getting another child into trouble it makes them feel better about their own shortcomings…
A final reason for chronic tattling reveals the deeper issue of children’s lack of independent conflict resolution skills. These children have no other strategies to deal with undesirable peer behaviors, and so look toward the adult to resolve the problem. Their limited skills necessitate intervention from adults, rather than relying on internal strategies to resolve peer issues. These children would benefit from peer mediation and conflict resolution training.”