SEATTLE — The omicron-fueled surge that is sending COVID-19 cases rocketing in the U.S. is putting children in the hospital in record numbers, and experts lament that most of the youngsters are not vaccinated. “It’s just so heartbreaking,” said Dr. Paul Offit, an infectious-disease expert at Children’s Hospital of Philadelphia. “It was hard enough last year, but now you know that you have a way to prevent all this.” Offit said none of the vaccine-eligible children receiving care at his hospital about a week ago had been vaccinated, even though two-thirds had underlying conditions that put them at risk —...