There's nothing sadder than seeing a mentally retarded child in a family of people who do not get that child the intervention he or she needs in order to live an independent life.
My dad taught special education and he had students who had been pretty much ignored until they were old enough to start school. The parents always seemed shocked the child was mentally retarded. He said by the time the child was five years old, so much therapy time had been lost that it was nearly impossible to make up the deficit. He said it was a tragic circumstance.