I think you may be asking in the wrong sub. Lying is understood to be a conduct associated with the development of the theory of the mind. When children realize that their mind is different but similar to others' minds (and thus that information asimetries exist), they also learn to utilize this to their favor. Children keeping secrets from adults is a fundamental part of their socialization, and obeys standard dynamics of ingroup and outgroup. You would do better by asking developmental psychologists.

There's the whole concept of prosocial lying, which is telling lies to benefit others, and it is present in children from early ages. Prosocial lying increases trust and group cohesion.