BS!
People during times of grief and duress and the unexpected loss of a loved one, a child or a parent, spouse, etc. one sometimes find moments of humor and even levity even in their grief. People deal with loss and tragedy in many different ways. Saying that someone who experiences such a loss shouldnt smile or laugh for years and that anyone who does is not sincere, is simply ridiculous.
As for what you said about finding humor and levity in one's grief, that may happen when a parent or adult who has been dying for a long time finally dies. But for someone whose life has been shattered by the death of a young child, there is no laughter for a very long time.