Children's tendency to seek out peers of their own sex and avoid peers of the opposite sex is referred to as gender:

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Gender segregation refers to the tendency of children to seek out and associate with peers of their own sex and avoid peers of the opposite sex. This behavior can be observed as early as preschool age and is influenced by a variety of factors, including socialization, cultural norms, and individual preferences. While gender stereotyping, role development, and discrimination may also be related to children's gender-related behaviors, they do not specifically refer to the tendency to seek out same-sex peers and avoid opposite-sex peers.

Children's Tendency to Seek Out Peers of Their Own Sex