Can't send Game Center Invites

Hello

My twins are trying to invite each other as friends in Game Center. We CAN play against each other through Battle Ship for kids. But we cant seem to add each other through Game Center. we click Friends ... + ... Type in the name (we have tried thier email and their game center name) the click send. Nothing happens. We have tried it from both ipads. We have updated our software. We have rebooted. We have logged out and back in again. I have turned of restrictions. ???? Please ****.

Thanks

Mom