Katymac, I think that until they can look you in the eye and tell you that they have ruled out all physiological and pathological causes of sudden deterioration of vision, then you are right to push them.
You have already been told that your DD is atypical, because they expect it to last weeks not months.
I personally agree with you. My mantra when it comes to DD1 (she has a known brain malformation) is that when I am old and grey, possibly sat in a nursing home, whatever, I don't want to look back and think "could things have been better for her if I did x, y, z? Did I write her off? Could she have had a better life?" I want to be able to reflect and think "I pushed, I strived, I did everything I could to make sure that everyone around her gave her the best shot at life".
I think that until you know that an MRI/CT is clear, then you will not be satisfied that it is psychological. And children don't die of psychological causes of sudden eyesight deterioration being misdiagnosed as physical. Sadly, some children do die because physical conditions are written off as being psychological, or 'growing pains' or whatever, that they would not do if that patient was an adult.