Boston Children’s Hospital trustees have reached inside the organization to name Sandra L. Fenwick, an operations whiz who has been the driving force behind reining in costs, as the next chief executive of the nation’s largest pediatric research institution.

Fenwick, 62, who is now president and chief operating officer, will take over the top job at Harvard-affiliated Children’s next Oct. 1 from Dr. James Mandell, the hospital is set to tell its 8,500 employees Thursday morning.