Board members of the tax-supported University Health System admitted they were taken aback at how quickly events had unfolded. On Friday, Christus Santa Rosa Health System announced that it would immediately begin transforming its downtown campus into a stand-alone children's hospital. Last week, the top-ranked Children's Hospital of Philadelphia, along with Vanguard Health Systems, which owns the Baptist Health System here, promised UHS and the University of Texas Health Science Center a proposal within 60 days. George B. Hernández Jr., president of UHS, predicted that his staff would be negotiating either with Vanguard or HCA, and that UHS needed expert help from consultants to represent the public interest in negotiations.