Thinking Sideways: The Beaumont Children

On January 26th 1966 Jane, Arnna, and Grant Beaumont left their home to go play on the beach. When the children didn't come home two hours later as expected their mother was unhappy but not overly concerned. When they hadn't returned home 5 hours later the family called the police. Though there were many witness reports of the children no evidence was ever found to explain what happened to them. Where did they go and, more importantly, who took them?