Life is full of ups and downs. Stability is contingent on our ability to go with the flow, to grieve well, handle anger appropriately, express emotions instead of stuffing, etc. I do not believe in sheltering a child from unhappiness, if you do they will get hit HARD at some point in growing up.
The child will be influenced and will learn how to handle difficult situations and unhappiness as the adults in their lives model it before them. If there were a job description for parenting; The need for the parent to model healthy emotions and handle the ups and downs of life should be at the top.