Roddy's accent
There doesn't appear to be an explanation in the story for Roddy McDowall's noticeable accent.
Living on a western USA ranch, Roddy has a noticeable British accent, while being the young son of an American family. There is also a smart ranch hand with a Scandinavian accent, who doesn't appear out of place in the story. This was filmed during the Second World War, and Roddy was still a young boy at the time.
There was a trend among British families to send their young children away from London to relatives living in more remote areas of Britain to keep their children safe during German bombings of London.