I'll give away my age here by saying that when I was a kid we didn't have a TV; not many families did at the time. We'd often listen to children's serials and plays on the radio. Many of the "child" characters were played by adult women. There were (still are) strict laws about how many hours a child actor could work, so it made sense to employ a woman. There was also the danger that a child could grow up "too soon" and their voice break during recording.