why does the nurse fear for medea's children?

Medea isn't exactly mentally stable. She's pretty angry that Jason is leaving her for the Princess of Corinth. That's understandable. Medea, however, becomes increasingly vengeful and depressed. She alternates between screaming for revenge and screaming about killing herself. Jason plans to exile Medea and the children while he takes up with his new honey. The nurse is scared for the children because Medea is going bonkers.