The worst thing you can do is trivialize parents' pain by telling them they can always have another one, or they should be thankful for the children they do have (if they do). Just as with anyone else who has lost a child or another family member, parents should be allowed to grieve and given time to make sense of their grief. It is helpful for them to know that Christ can redeem their painful experience, but please don't try to move them to that point.
