one of our twins lost their voice for a few days when sick, a month before going to WDW... we told them that they were sick, and they totally bought it.
HOWEVER, the CM that works with the mascots told them that if they talk to one child, they have to talk to ALL the children, and they would get tired and their throats would get sore, so they just choose to save their voices for the parades etc. One of the boys quickly told the CM that this wasnt right, it was that Mickey was sick and had lost his voice. This is when they were 2.
They asked every time... but then quickly remembered the "reason". I would even THINK about telling them they were people dressed in costumes. Let the magic live!