Is the voice of a woman who adheres to the prescribed Islamic dress and does not speak alluringly considered 'awrah when giving lessons in public places as part of da'wah work directed at children and their parents?

A woman's voice is not considered 'awrah, and it is permissible to listen to it as long as there is no softness in speech (khudu' bil-qawl) or it does not lead to temptation (fitnah). However, a woman must observe hijab (definition) in front of a pubescent child who is nearing adulthood.