Children's Hospital in Pittsburgh is suing the mother of 7-year-old Ronnie DeSillers to collect $261,189 owed for his three liver transplants, and a hospital official said the woman apparently also has not paid doctors who operated on the boy. In Florida, Maria DeSillers said she is withholding payment because she has "a lot of questions" about the care given Ronnie, who died April 29 while awaiting a fourth donor liver. People around the country had donated an estimated $500,000 for his medical expenses, including a check for $1,000 from President Reagan. The hospital sued DeSillers in Allegheny County Common Pleas Court after she failed to respond to repeated requests for payment, Edwin K. Zechman Jr., hospital president, said. The last appeal was made a few weeks ago and gave her 14 days to respond, he said.