Like any parent, Brittany Young has scores of photos of her four children, whether they’re celebrating the holidays, visiting Disney World or just goofing around. But it’s a picture of her son Kellen at Boston Children’s Hospital that has the potential to bring tears to a viewer’s eyes. Surrounded by his care team, Kellen sits in his hospital bed, a wide smile on his face. There, amid the stuffed toys and colorful blankets, rests a seemingly innocuous item: a package of Pampers. Yet for Kellen and his family, those diapers mean so much more. They’re a sign that this little boy’s newly transplanted kidney is working just as it should. …