I wonder what Montessori nursery your child is attending, as I could have written replies to your posts as the mother of the other child.
DS had a bad start in his Montessori school. Even when I insisted to the staff that he was perfectly able to communicate in English, they decided that his shyness was actually lack of language skills. DS, who is not stupid, found it highly comfortable not being asked to speak and at some point I realised they had been communicating with via hand signs! .
The teacher was also concerned that he was not mixing up with other children, decided she couldn't see any imaginative play, and voiced her concerns. What she never said was that she was a cow to him, she never tried to integrate him, she tried to push him into doing what she wanted by force rather than trying to get his interest as she did with any other of the pupils. My son was the only child who was never conforted if he cried at the start of the day. I just kept him in the school because I have great faith in the method and because I knew it would be only a few months before he got a new teacher.
During the holiday club, and away from that teacher, DS flourished, he became more confident and developed a strong friendship with another bilingual kid. And they started to play up...
By the end of the first month in reception the headteacher wanted to speak to us as they were concerned, again, about "his development" and ability to relate to his peers.
So...I bounced the ball back to them and asked them what they were doing to make him feel accepted as it looked as they could only speak negatives about him. I also asked how they were planning to deal with the bullying as if my very placid DS had taken to be unkind there should be a good reason behind it. I also took the report of previous nursery and made a point saying that the word that was normally used to describe DS, by every single person who had taken care of him was "placid" and asked them how such a placid, kind and well behaved little boy could be acting in such way in such an outstanding school. And agreed for DS to be seen by the SENCO.
A few days later we were told that the SENCo had said she couldn't appreciate any problem in the language skills or development, but may have said something about him feeling rejected because they changed the way they dealt with him and since then everyone is happy, from DS to the teacher and obviously ourselves.
Obviously, DS is still his distracted self and copycat so from time to time he gets into trouble, but in general he is doing much better than before and we are, now, very pleased with the way things are going.
So, what I am trying to say here with all this babble is that the SENCO being involved is not a negative thing. If he has a problem they will help him, if he hasn't that will help him as well. At the time I was desperate to take my son out of the school and tbh the only reason I didn't do it, was because they were BRILLIANT at dealing with food allergies and because I knew that sending him to a bigger school, where he would not even get half the attention, was going to make the things worse.
Now that the problem is sorted and DS is getting better and better I am very happy we decided to keep him there. The school is fantastic and DS seems very very happy. We are very pleased with their work.