Freud mentions more than once that the common assumption that sexuality is absent in human children until puberty is an error:

One feature of the popular view of the sexual instinct is that it is absent in childhood and only awakens in the period of life described as puberty. This, however, is not merely a simple error but one that has had grave consequences, for it is mainly to this idea that we owe our present ignorance of the fundamental conditions of sexual life. A thorough study of the sexual manifestations of childhood would probably reveal the essential characters of the sexual instinct and would show us the course of its development and the way in which it is put together from various sources (39).

For his part, Freud argues that childhood is actually fundamental to the development of human sexuality. Conversely, childhood sexuality is fundamental to the psychological development of a human adult.

According to Freud’s psychosexual theory of development, several distinct phases of sexual development are experienced in infancy and early childhood. Infants start by indulging in what Freud calls autoerotic acts, such as thumb-sucking (74-75), which is a means by which infants recreate the experience of breastfeeding when they are left alone.