The coronavirus pandemic is upending everyone’s daily routine, including an 89-year-old man who donates his time to help sick children in hospitals.For many years, Bill Brennan, who will celebrate his 90th birthday in two weeks, has cheered up patients at the Dana-Farber Cancer Institute and Brigham and Women’s Hospital in Boston with stickers.While volunteering at Brigham and Women’s, Brennan met Nancy Kleiman, the hospital’s harpist,