All those parents and children who will be attending the last day of the Ringling Bros. and Barnum & Bailey circus at the D.C. Armory Monday are in for a special surprise -- President Reagan is planning to show up and deliver a speech on "lost youth." The White House says there will be an announcement today. And all those kids thought they were only going to see clowns, elephants and high wire acts . . .

Romuald Spasowski, Poland's former ambassador to the United States who defected in 1981 when martial law was declared in his homeland, became a Roman Catholic Tuesday. The former atheist was baptized in Philadelphia by John Cardinal Krol and said he would dedicate his life to the church and the Polish people . . .

French pianist and composer Gilbert Be'caud ("What Now My Love," "Let It Be Me") finds it difficult to perform on stage behind a piano because it is difficult to see and be seen by the audience. When he performs at the Kennedy Center Monday he will be playing a piano he had constucted to resolve that problem. The new piano, which he has brought from France for the tour, is made of clear plexiglass, with sparkling solid gold fittings . . .

Singer Jerry Lee Lewis, a man who seems to live under a black cloud, collapsed during a country music festival in Belfast Tuesday. He was reported to be better yesterday and his illness was blamed on exhaustion and food poisoning . . .

Washington writers Bob Woodward, an assistant managing editor of The Washington Post, and Christian Williams, a Post investigative reporter on leave, have written a treatment for a screenplay of William L. Shirer's best-selling autobiography, "The Nightmare Years." Woodward and Williams, who interviewed the 83-year-old Shirer at his home in Lenox, Mass., about his years as a broadcast journalist during the rise of the Nazis in Germany, will be preparing the screenplay for a television mini-series produced by Gerald Rafshoon, Jimmy Carter's former media adviser, and Los Angeles producer Bill Finnegan . . .

Motion Picture Association of America president Jack Valenti has just returned from India where he was signing a renewed film treaty with the Indian government. He met with Prime Minister Rajiv Gandhi and told the prime minister he was a B25 pilot during World War II. The prime minister, who was a pilot with Indian Air Lines, lamented, "I did nothing as exciting as that." To which Valenti responded, "You would have if you had landed at JFK at 6 in the evening" . . .