There is no reason to think that Neanderthals had any disabilities. They were extremely successful for tens of thousands of years and adapted to living in a very cold Northern Hemisphere. They were a subspecies of human not sub human. For a very long time homo sapiens was around and not doing anything significantly different to the neanderthals.
I do suspect that they were less aggressive though and that homo sapiens saw them off for this reason. But it could have been an oscillating climate that caused a population crash.
I also see no reason why a neanderthal child could no be brought up successfully - the problems would be more likely to stem from other people knowing, than the child itself. Although they might reach milestones of development earlier (??) The appearance of modern homo sapiens is quite diverse and there would I think be an overlap area in which it was impossible to tell the difference in facial features. Many people have a neanderthal body type - stocky with a wide flaring ribcage, including my own DH, and this might be a direct result of neanderthal genes in our population.
But the most fascinating think about neanderthals is what their culture and language was like and that is something that would not be revealed by such a pointless experiment as the one suggested.