why do you think children keeps friendship with same gender.?

It boils down to common interests. Boys relate to manly issues and the things that help them acquire manly status and usually the one to help them get there on life's journey will inevitably be another boy. With girls it is the attainment of femininity and another girl helping the individual concerned getting there. However there are occasions where opposite sexes may establish strong platonic relationships which if go unchecked by the parents of both parties concerned may result in more grown up interests that both may not or shouldn't be ready for at that point in their lives(ie. sexual interests). Or the end result being either the boy becoming more feminine or the girl becoming a Tomboy of sorts.

Culturally simulated as well. Girl's rooms are pink, boy's blue. Parents will often put their kids in sports: boys- hockey, soccer.. girls-dance, gymnastics and they will meet others like them there.

I'd say that most girls can relate to each other.. if they have a problem, it's likely that another girl may have had the same problem.