Could it be becuase of the privacy regulations and HIPPA?
Both my parents are doctors and I have worked in their offices in the past. They both had private 18-and-older-except-by-special-circumstances practices. While most children are well-behaved, there are those who are not.
I can clearly remember one father bringing his two children
and three of their friends in for his appointment. He left them in the waiting room becuase he didn't want the children in the exam room. It was no big deal visit as we saw him every month the day before his RXs ran out

I'll lave it to you to figure out which type he was refilling so regularly. As soon as he got out of the waiting room, the kids became a ghouls. They were terrors.
Absolute terrors. When I knocked on his exam room door to ask him if it would be all right if the children joined him in there (it was a large room and it was better then them upsetting the other patients), he told me to, "Deal with it." Never in my life...
While I can understand the physician's reasoning as it is a
professional medical pratice, it does seem odd to me not to provide some measure for the parents to provide care of the other sibling(s) when it's a Pediatric practice. We don't live in a country where the norm is a single-child family.
I will be interested to see if this stays in place if it is an arbitrary thing on the physician's part and not a Fedral mandate.