This verse is used to support that children, when they die very young, before they have a chance to understand sin and the Gospel, get to go to heaven.  Jesus seems to confirm it.

          The same support that this verse lends to children, should apply to the mentally handicapped.  They have child-like minds and many do not have the capacity to understand their actions, understand God or exert their freewill.  If the physical wiring of their minds overcomes their sense of self, can they be selfish?

          Does this mean that all mentally handicapped people get into heaven?  It depends on the severity of their illness.  For example, Autism has degrees of functionality.  Also, some mental challenges occur because of one's own sin, like drugs.

          It's also risky to say that if someone had an accident that caused their brain damage whether they get an automatic pass into heaven.  If they refused God before the brain damage, only God would know if they would accept him if the accident never happened.  That's a tricky call that we must leave up to God.

          While we can't blindly say everyone with a mental challenge will go to heaven, we can rest assured that God will take their illness into consideration and judge them according to their ability and disability.