The boy's 14-year-old sister was seriously injured as she too fell off trying to save her younger brother.
The girl was hospitalised at Thiruvalla in the district, police said.
The incident occurred last evening at the carnival centre when Alen (5) and his sister were sitting in a 'jhula', at a height of about 30 feet from the ground, police said, adding that as the wheel gained speed, the boy panicked and was eventually thrown off the 'jhula'.
The two kids had come to the carnival along with their parents, police said.
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