I think you are right to be concerned. However If you are employed as a Nursery Nurse then you need to prove that you have the necessary qualifications so if you are relying on your teaching qualification for this I suppose you need to show the appropriate piece of paper. I wouldn't have thought that meant showing your teacher ref. number (is this what used to be your QTS number?). Why don't you just ask what it's needed for and go from there, I can't see how they can refuse to tell you.