Mary Mary performs for a packed ballroom at the historic Willard InterContinental Hotel during “A Cinderella Ball,” an annual event staged by The House Inc. for children of military families who have disabilities or life-threatening illnesses. - Mary Mary performs for a packed ballroom at the historic Willard InterContinental Hotel during “A Cinderella Ball,” an annual event staged by The House Inc. for children of military families who have disabilities or life-threatening illnesses.