SaliMali1, with the utmost respect to you, you need to be careful about sharing information about children in your care on the internet. You are being given access to confidential information purely because of your role as a teaching assistant.
While I fully understand and applaud your conscientiousness with regard to the child in your care, I would be most upset if I thought the teachers or teaching assistants involved in my DD1's care were talking about them on an internet forum.
By all means investigate and ask questions, but at least attempt to cover the fact that this child is in your care. Ask questions in a more general way, or write it as if it were your child, for example, to distance yourself from the real life situation.
SEN Assessment Units exist to help get school right for children with complex SEN where the right placement isn't immediately obvious. The staff there get a very good idea quite quickly of what the best setting might be for a child.
Of course, the parents, in 99% of cases, can INSIST on a MS setting for their child, but they will realise (sometimes too late) that MS places are very easy to come by. 1:1 support is cheap, and many parents aren't knowledgeable about making a statement watertight, and their children can end up with very average provision. Some will have the benefit of sites like this, and have better statements with well quanitified provision, but this not common.
There are huge benefits for some children to be in a Mainstream setting with Mainstream peers despite being cognitively far behind.
There are huge benefits for some children to be in a Specialist environment (ie. a 'base' within a MS school, or a Special School).
What you can be absolutely sure of is that it is very unusual for a child to be offered a Special School place unless they need it.
DD1 is in a class of 10 children at her Special school. Of the new starters this year only one is coming from a Mainstream pre-school into the Special school. All the others have come from a SN nursery. The one child is DD1, and it is full credit to MN Special Needs: Children posters that she has that place. She wouldn't have it if I had listened to the advice of professionals who are involved in her care.
I would consider very carefully what is wisdom in this case. To well-meaningly suggest that the parents fight this suggestion without allowing the child to settle in the Assessment Unit and everyone consider what is in the child's best interests could strip them of a valuable educational resource.