I don't think points 2 and 4 are necessarily true. There are 7 year olds who are anything but nice; based on the information, however, I can't say if this particular 7 year old thought they were being nice or not. As for point 4, I think it is offensive but in the way that even well-meaning words can be offensive when filtered through an ableist worldview. The child probably doesn't know any better, and the parents aren't teaching social justice at home. For someone who wants their child to feel like they "belong" and are the same as everyone else (never mind if this is desirable to anyone else), it must be frustrating...
As for the mother being angry, it is her right to be angry at whatever she wants. Is it productive? I don't know; doesn't it depend on what she wants from her anger? Maybe she thought that the parents of the child should have taught them to be fully accepting?
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We have to change our way of thinking if we really want to change the future. - Saki Watanabe (Shinsekai yori)