Generally I taught girls that it was okay to hold an arm across their face and chest if they wanted. I also taught how to tuck the chin and take a glancing blow off the head like slipping punches when a ball is coming at their face..
You can't make it easy for the official to call a hand ball on them if they move the arm. With boys one hand over the jewels. That is the way I learned. But now we show them take the other arm and touch the top of your far shoulder. You will see you got your fore arm and elbow protecting your face and nose. You kind of look like Bela Lagosi when he played Dracular. Then if they blocked the kick we go wild with praise.