Response: This is a very difficult and sensitive issue, and I hope I can offer something helpful in response. The fact that you would ask the question tells me that you are a person who cares about others--especially the weak and vulnerable. So, the first thing I want to say by way of response is that God also loves and cares for all people, especially the weak. He cares so much in fact, that He became weak Himself, taking on flesh and subjecting Himself to suffering and even death, in order to save people who are not just weak, but completely helpless to save themselves (myself included). We may not always understand why God allows suffering, but the shadow of the Cross looms over history so that we never have to wonder, "does God care" or "will He do something about it?" He does, and He has.
Secondly, I think it is worth pointing out that the question carries with it a sense of anticipation. That is, we intuit that there is only one answer that would be morally justified. We sense that it would be wrong for God to condemn severely mentally handicapped individuals to Hell. Our hearts are ready to leap to the defense of the helpless. Now, it is important to note what exactly we are longing for in this situation. We want God to do the right thing. Granted, we desire a particular result, but that is only because we are convinced it is the only just option.
Even though we cannot comprehend everything that God does, we can apprehend certain things about His character--especially when they are made explicit in Scripture. One of the things that is most clearly seen in Scripture, and by philosophical reflection, is that God is perfectly just. His nature is the very standard of justice. God never acts out of step with His nature; thus, He never does what is unjust. Peter tells us that when Jesus himself endured tremendous suffering, He entrusted himself into the hands of the perfectly just judge (1 Pt 2:23). I believe we can do the same with our friends and loved ones who are suffering with severe disabilities. We can trust them into the hands of the God who does no wrong.
*This question came from a real person, but was not originally directed toward me. Even so, I wanted to offer my response in case it is helpful to anyone here.