Boys are friends with boys and girls are friends with girls.
This trend really starts in first grade and it continues in second grade. Girls form many friendships, but they are often fickle. Boys have smaller groups of friends, but their friendships generally last longer.
What you should do: Don’t do much. Don’t worry if your child still has a best friend of the opposite sex. The only time to worry is if your child stops trying to make friends altogether.