Is it potentially harmful for a five-year-old to learn about the menstrual cycle?

Depends: It sounds a little young. Sometimes the sight or thought of blood upsets little ones. Use good judgement.

Harmful ?: The average 5yo is caught up in the virtues of the tooth fairy & santa & is not capable of processing & understanding the events of the world to any depth.They should receive short simple answers if they question you about menses but avoid the temptation to begin a class on body functions. Save it until the tooth fairy is dropped & kid asks more direct questions.Then your efforts may stick.