I wouldn’t call any child, as a human, a disappointment, but I believe there are some rare occasions where their actions should be framed as disappointing.

Label the action disappointing, explain the reasoning that led to that conclusion, and explain how it could affect the future for both the parents and the child. Communication is key, and also try to leave some room for the child to grow. The less often you call something disappointing, the more powerful it can be, and can be used as a way to seriously correct behavior.