How do you get children to put their heads under the water?

I try to tell the kids everything will be ok and to breath through their mouth and nose. I even play games like ring around the rosies and humpty dumpty but they still wont go under! Even if I show them how to do it, by me doing it they still wont. Please any suggestions or if you've been a swim instructor and have any ideas that you did that worked that would be fantastic! Thank you!