Its hard to see someone hurt so badly after a loss and it is natural to want to make the hurt go away. Unfortunately nothing you say or do is going to make that happen. Grieving is a natural process that we go through to help us accept a loss, especially one as devastating as a child. The parents can no longer dream of firsts, graduations, weddings and grandchildren from that child. They will deny, bargain, feel anger, and finally accept.
Don't say: You can always have another another child will not replace the one who died
Don't say: It was Gods will or It was for the best no one knows Gods will, and no one can say for sure what is best in the big picture of life.
Do: Listen to them and don't be afraid to talk about the child, using the babys name
Talking about the baby is not going to remind the parents of their loss.
They are already thinking about it
Do: Remember the child's birthday, the due date, etc. and holidays (these will be especially hard)