Is it ok for kids to only have friends in their own religious group?

Hard to say: Some religions strongly prefer to keep their children close so that they can protect them from outside influences and ensure that they receive the training and experiences the religious leaders/parents want for them. It seldom is fool-proof and not infrequently leads to rebellions in adolescence or young adulthood. It's nearly impossible to keep other points of view from cropping up.

I say "not okay": There are several cults that actively prevent children from making friends outside their communities. One in particular in the us is detested for preventing teens from taking part in sports or after-school activities. In today's world, being an effective adult involves relating to a great many different kinds of people, and choosing one's friends freely. And teens bitterly resent the interference.