Inhumane for Who?

CNN has a very interesting article here about a radical treatment option the parents of a girl with cerebral palsy took. Instead of the traditional route of "let her grow up and we'll continue to take care of her" they decided to stop her growth. As a result, she's forever about 4.5 feet and will never sexually mature. The question is if this is something they should have done.
On one hand, you have the slippery slope argument, where if the parents of one mentally challenged girl do this and it's legal, what's to stop people from doing it to every mentally challenged child? What about other children? Isn't that what Hitler did? I can see that argument and agree with it somewhat, but at the same time I say that it's a big leap from stopping the growth of a child with the brain of a 6 month old and the Nazi application of natural selection.
On the other hand, you have parents who definitely care about this girl trying to make it possible for them to care about her. She'll never get too big (well, too tall) for them to care for and they won't have to worry about someone raping her and getting her pregnant. I think the immediate reaction is "who are they to play God?" but the more that you look at it, the reaction should be "what is society to play God?" The parents clearly care about her - they're not sending her to a foster home or a hospice or long-term care facility, in fact they did it so they wouldn't have to do that. In the end, if parents are caring for their child and doing what is necessary to not pawn them off onto society but rather care for them, isn't that what's most important? The parents love this girl and are doing what they feel is best for her as she can't make the decision herself. As far as I'm concerned, that's their right as parents and society should butt out if it thinks they're wrong.