Ok I have a feeling my reply to this will not make any sense but I'll give it a shot.
Changing the physical appearance of a child with DS (by which I mean the facial malformations that come with it), is not something I agree with because it will do nothing to alleviate the problem, which is actually a mental one - it just happens to have a physical manifestation, which by all means you can alter but it doesn't stop the kid from having DS and thus nothing is gained other than a mild superficial improvement, and I say mild because like katie said before, the deformities are really hard to actually fix.
If the DS kid in question got burned, I would agree that surgery would be an option, because the benefits are numerous enough to outweigh any argument about the child's mental capacity. Pain relief, mainly, which I assume is the main motivator behind most reconstructive surgeries of this kind, as well as the fact that, as horrible as it might sound: the kid was born with 'abnormal' physical characteristics that are hard to disguise anyway. If horrible burns are added to that, that's just unfair. They weren't born with it, so why forgo corrective surgery for it? They're only going to look worse than they already did. You may as well try and fix the burns because that'd be easier than restructuring a face and would probably have a better end result.
Does that make sense?