A preschool child tells on other children to exert his power and gain favour with parents or teachers. A child often learns that telling tales gets the parent or teacher to make things happen. For instance, when a child tells his mum that his sister took his toy and called him a name, the mum will often punish the sister and give the toy back to her son. By punishing the child who is not telling tales, you are reinforcing the behaviour by helping the tattler get what he wants. If however you refrain from getting involved, your child will eventually realise that he must work out the problem on his own.
Another way to address tattling is to increase the cost of telling tales. If your child tells on his playmate, give him some work to do. As an example, you can tell him to draw a picture of what happened and you will look at the picture later. Putting the burden back on his shoulders may make him decide that it’s just not worth the effort to tell.