Slightly different perspective.
My child is in state secondary school. He struggles with EBSA. He can't articulate why he is anxious about school, and the school is super supportive. Academically, he is actually quite bright.
However, a problem last year was that he found classes to be noisy, and this upset him. Also that there were some disruptive children and that it upset him when there was little actual work done in classes due to disruptions etc; he gets indignant about being set homework when he feels time is wasted in lessons. In Science, he was deliberately sat next to someone struggling as a "good influence". He complained to me that one child kept taking his things and tried to break his calculator.
In discussions with teachers about increasing his attendance, it was admitted he was in a "noisy class", with additional support due to SEN kids. They were understanding that he might be finding it difficult, and tried to support him as best they could.
This year he is in different groups for learning, with some streaming, and that has helped him. I am all for inclusion and being understanding, but not at the expense of my child's education.
So, I am not sure you can do anything right now, but I understand your concerns, albeit from a perspective of a different age group and circumstances. In your place, I would try to keep an open mind but keep checking in with my child to see what they are experiencing in class, without asking leading questions, and to see if there are staffing issues or decisions that mean the presence of the new child is interfering with your child's education. If what you hear concerns you, then you can focus on those concerns WITHOUT explicitly attacking the new child's inclusion. For example "my child is finding class to be very noisy / keeps getting told off because you have sat them next to someone who keeps talking to them at inappropriate times or needs help from my child" etc.
And, regarding the age gap, I think people have been harsh. It is something else to keep an eye on. They won't stay 5 and 7, but will be 6 and 8, 7 and 9, etc. That is close to the age gap of my kids, and my eldest can be more powerful physically, and had a smartphone first, and played more mature games online first, and moved away from playing with toys first. You want to check your child is getting a chance to be a child in an age appropriate way, and not getting dragged into older stuff due to (peer) pressure from an older child.
Finally, the fact they are a teacher's child would also be causing me to pay attention. From experience, this can lead to favouritism and/or leniency.