I did a blog posting on that here:
But this is something you may not have known. In 1940, Queen Elizabeth, then just Princess Elizabeth, also made a Radio broadcast - and the details are given here, from the BBC Handbook of 1941:
The most notable day in the year's broadcasts to children was Sunday, 13 October , when Princess Elizabeth broadcast a message in the Children's Hour to children of the Empire. This was Her Royal Highness's first broadcast.
Princess Margaret, who was at her sister's side while she was speaking, also made her maiden speech over the air by bidding their listeners good-night.
The speech was recorded for inclusion that same evening in the first of the new weekly programmes for children evacuated to Canada and the U.S.A.