No.  Absolutely not.  Children should experience unhappiness.  They should experience disappointment, loss, and sadness, frustration, etc. because these are all inevitable and healthy life experiences.  If children are sheltered from any of this, they will struggle with learning how to navigate it all. Then, they may become self-centered, narcissistic adults who are not capable of solving their own problems are handling a dispute with someone who doesn't agree with them.

Children's jobs are to learn how to be healthy and productive adults.  In order to do this, we adults have to give them safe environments to practice the skills that it takes to be a healthy and productive adults.  Dealing with unhappy and disappointing situations is one of those key skills.