Nurse Isabel Amaro, 47, from Manchester, was found guilty of manslaughter after three days of deliberations at Nottingham Crown Court.
Jack Adcock, who had Down’s syndrome, was taken to Leicester Royal Infirmary in February 2011 suffering from vomiting and diarrhoea and later developed sepsis.
Dr Hadiza Bawa-Garba, 38, and nurse Theresa Taylor, 55, both of Leicester, are also facing manslaughter by gross negligence charges.
The jury will continue its deliberations on Tuesday. All three denied the charges.
Jurors have heard three weeks of evidence about the staff members, who prosecutors alleged breached their legal duty of care to Jack, from Glen Parva, Leicester, in a series of gross failures in his treatment.