I would reply with something like, "thank you for your detailed letter. I have forwarded it to our psychologist, and (s)he will be in touch with you to discuss. I will also be raising your concerns with his in-room carers."
Talk to those who actually look after him- show them the letter and see their reactions. Get your psych involved and say this is stressing you and could he/she please speak to the centre leader directly.
I agree that if your DS really dislikes going there, it's time to move on. BUT, just in case (sorry, the childcarer in me is careful) make sure the psych gets the letter to put your mind at ease.
Also, unless she is a qualified psych or paediatrician or development specialist, etc, she has no right (or grounds) to do an assessment. I would also mention this to your psych.. perhaps the psych can show her how out of line (and bordering on fuzzy legal grounds- she's not a dr!) she is.