It's an important question, and one which churns around in a person's mind as they wrestle with the right words to say. What can you say? Words of comfort, of course: but which words?
It seems that, nine times out of ten, all a bereaved parent wants to hear are words. Just words. A sentence of acknowledgement of the terrible loss that has occurred; a comforting hand on the shoulder; a simple 'I'm sorry'.
Others may talk too much, and try too hard to find a sentence or phrase which will instantly make everything better. Intentions are often good, but the words come out wrong and be quite damaging.