SIMI VALLEY, Calif.—Locked in a war room with military officials shouting at each other about the impending invasion of Grenada, Gen. John Vessey, President Ronald Reagan's Chairman of the Joint Chiefs of Staff, rose from his chair.

"People! People!" he shouted. "Gen. Vessey has a request: I am super thirsty."

His military commanders rolled their eyes and resumed the debate. Gen. Vessey—who outside this room was 13-year-old Christian Graves—slumped in his swivel chair, sighing deeply. He then ordered Army Rangers into Grenada.

In a corner of the Ronald Reagan Presidential Library and Museum, beyond stately White House portraits and a sizable chunk of the Berlin Wall, Ronald Reagan's legacy is playing out in an unexpected way.

On multimillion dollar sets replicating the Reagan White House, children play the parts of key officials and reporters to reenact the invasion of Grenada. The U.S. invaded the Caribbean island nation in 1983, fearing a communist takeover after a coup.