Coming after a day of sightseeing them that took the family to Westminster Abbey, the pre-birthday party settled into special seats in a private box at the Lyceum Theater in London's West End, where the Disney stage musical is playing.
The group quietly sat down after the houselights dimmed. But it wasn't long before their fellow theatergoers – some 2,000 of them – spotted the girls enthusiastically sitting on the edge of their seats and leaning over the ledge to better view the stage.
"The children were clearly having a good time," one fellow audience member tells PEOPLE. "There was quite a buzz in the theater once people spotted them during the first part."
At intermission, people took to their cell phones to inform friends of the special guests in the theater. Afterwards, "I saw the girls skipping back into their box from their private room," adds the audience member.
Westminster Abbey ClosedThe Westminster Abbey visit started around 10:30 a.m., with the Gothic church closed to the public for about 12 minutes so the Obamas and their entourage could tour with the Dean of Westminster, the Very Rev. Dr. John Hall.
He showed them the "usual treasures of the Abbey" seen by tourists, says a source. This included a statue of the Rev. Dr. Martin Luther King Jr. which stands above the west entrance to the Abbey. They also saw the Tomb of the Unknown Warrior, the Coronation Chair and the Lady Chapel of Henry VII.
The Obamas have been in London since Monday after first visiting Paris.