According tο St. Paisios , parents should have joy for their mentally challenged children. Here is why.
Parents who have retarded children, must not feel sad for them, because their souls are already saved. On the contrary, they must be happy, because their children, without making any efforts, have earned paradise. What else can parents wish for their children? If they face their child's defect in a spiritual way, they are also benefited and rewarded by God.