Adults who have figured out that they have fear of microphones are often the introvert ones. They are so shy they do not want their voices heard. They are also the ones who were brought up in an atmosphere of minimized or no social life at all. When visitors come over, they were sent to their rooms.
Do you want your kids that way too? If not, help them by getting them used to one of the tascam microphones perhaps. Sing together. There are many videoke and karaoke videos over youtube which you can enjoy. That way you develop the confidence in your chilld.