Children's Hospital Boston has agreed to limit increases in fees it charges Massachusetts' major health insurers next year as part of a larger push to control the rise in pediatric healthcare costs. In exchange, the three health plans—Blue Cross-Blue Shield, Harvard Pilgrim Health Care, and Tufts Health Plan—along with the state's Medicaid program, will contribute a portion of their savings, about $10 million in total, to a fund that will enable Children's Hospital to accelerate pilot programs aimed at providing better care at lower costs.