Parent of a DD with Down syndrome.
For your answers about children with Down syndrome - spanking will work just about as well with a child with Down syndrome as it does with "typical" children. The outrage should be at the magnitude of the punishment of any child, not just the child with Down syndrome.
Yes, Ds does affect the speed at which a child learns, however they do learn. And their social emotions are much the same as their peers. Most people with Ds are only mildly to moderately developmentally delayed. As more and more inclusion and better schooling happens, and better medical care, it is being found that people with Down syndrome are way more capable than anybody thought even 10 -20 years ago. Of course, just as in a typical population, the abilities of the children are going to vary widely. But statistically, the typical bell curve has the majority of people with Ds only mildly to moderately delayed, with a few on either end; very involved to normal intelligence.
That child with Down syndrome was just as terrified and was experiencing exactly the same emotions as any of her classmates would have been if they experienced the same punishment. That part of their psyche is not affected at all.
That said, there are many parts of the law that protect a child from abuse.
One thing is that children with Ds often has speech delays, so although they know perfectly well what is going on, they often cannot verbalize it. So, people with Down syndrome are sometimes abused because the abuser believes they will not be able to tell. So, a harder spanking, more punishment, etc.
If the punishment was out of line, I would report it to child services.