Child's sex development can mean the development of gender identity and gender typification or the development of sexuality of a child. Therefore, education in sexuality should be distinguished from gender-adjusted education that is aimed at healthy psychological development through the adoption of gender identity and the formation of gender roles, while breaking gender stereotypes. Development is a complex process from the conception to the end of developmental age. The process of fertilization and chromosome binding determines the genetic sex of the embryo, which is followed by the sex differentiation of the body, accompanied by sex differentiation of the brain. Gender typing is a process during which children learn behaviors and acquire beliefs that are considered appropriate for their gender. Biological, cognitive and social mechanisms are involved in this process. Most children identify themselves as "boy" or "girl" by the age of three, ie they express their sex or gender identity. Gender stability occurs around the age of four, and gender consistency around the age of five. Gender stereotypes represent simplistic but deeply embedded assumptions about masculine and feminine traits in a society, and not only beliefs about what men and women are like, but also what they should be like. These stereotypes are quite developed by the age of five, and become rigid between the ages of five and seven, making the period of preschool education a critical period for dealing with gender stereotypes. In the empirical part, my own research on a group of preschool children confirmed assumptions about the process of gender typing that leads to the choice of gender stereotypical toys and clothing, gender segregation in the preferences of choosing peers for play, and the intention of choosing gender-specific occupations for their future. The possibility of stereotypes leading to sexism and gender discrimination has been disproven, because children believe that both sexes can be equally successful in all occupations.