He was unconscious when deputies and paramedics arrived, Denton County Sheriff's Sgt. Roger Griggs told the Denton Record-Chronicle for its Sunday online edition.
The boy had been at a water park with his parents, siblings and a family friend. Each parent drove a separate vehicle and apparently thought the other took the child inside after returning home, Griggs said.
The child was taken to a Dallas hospital and later died, authorities said.
Officials were investigating and have not released the boy's name.
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