I believe that we should, on biblical grounds, tell all parents of mentally disabled children that God loves their children, regrets terribly that they are disabled, and will, when they die, carry them gently into a heavenly life where every person is forever whole.
Some people still make promises and keep those they make. When they do, they help make life around them more stably human.
Any moment that opens us up to the reality that life is good is a parable of the supreme end for which we were made.