If your child is sad and stressed, is that alone serious enough to see psychologist?

Depends. This depends on several factors related to possible circumstances, how long this has been going on, and whether the child's normal functioning has been significantly affected. If symptoms of depression have been going on for some time, even with good reason, then it would be worthwhile to set up an evaluation with a mental health professiona. A good physical evaluation is worthwhile as well.
Feeling blue. Starting with your primary care physician is key, since they know you and your child best. Other symptoms including poor sleep, weight loss, declining grades or other behaviors and length of time it has occured is important. Counseling is always a good option and can be instrumental in providing a safe place, coping skills, as well as defining the possible need for medication.