The major point you are missing is that it is not for the "convenience sake only". The ability of the parents to care for this child will be enhanced by her smaller stature. You also fail to realize the mind of this child and the environment she sees. Her happiness is simple--close physical contact, being held, kissed, all of these things will continue to be possible.
The ethics board, far from perfect I'm sure, found this reasonable because in their judgement would result in a higher quality of life.
As far as you "calling it as you see it", what expertise do you have in this area?
So then I suppose you would support the parents if they chose to amputate her limbs since she would be unaware of her condition as a result of her "simple" mindset. I believe this same argument was used when Oliver Wendell Holmes wrote that compulsory sterilization of "imbeciles" was constitutional in the case Buck v. Bell.
My "calling it as I see it" was in reference to Nazis. One of my majors in college was history. Is that expertise enough for you?