In England (and other parts of Europe) a 'jumper' is what we Yanks would call a loose jacket, a vest or a sweater.
Now, if the context of the story is that people are concerned that a Christmas sweater might upset a youngster, I can see why -- some of those sweaters are downright bug-ugly, and I would not be caught dead wearing one.
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The mere fact that science may not yet adequately explain an object, event, or experience does not mean the immediate explanation should automatically default to a conspiratorial, extraterrestrial, paranormal, or supernatural cause.