Riley Burton, sunny and full of giggles, sits on her bed in the Children’s Hospital of Philadelphia and flaunts one of her well-known grins. She takes a break from singing “Let it Go” from Disney’s “Frozen” to chat with pediatric oncologist Tracey Jubelirer. The pair talks about dolls and playing dress-up while the doctors run their tests and check her white blood cell count. Afterward, Riley colors a butterfly with a purple crayon, relaxed.