KYIV — Bubbles shimmered in the air, floating peacefully, until they met the vengeful finger of the 4-year-old with the bald head.

       Pop.

       He giggled — all bright eyes and chubby cheeks — and leaped again, chasing the sudsy apparitions across the third-floor hospital room where he’d spent half his life fighting cancer. The pink-wigged, bubble-wand-wielding fairy cheered him on — “Catch them! Catch them!” — as a line of medical staffers clapped.