Non-alcoholic fatty liver disease (NAFLD) has increasing prevalence in children and early risk factors for NAFLD may be present during infancy. MRI can non-invasively quantify hepatic fat, but current techniques require breath-holding (BH), which is not possible in many children and infants. In this study, a novel free-breathing (FB) 3D stack-of-radial fat quantification technique was developed and evaluated in children and infants. The proposed FB technique achieved accurate and repeatable hepatic fat quantification and improved image quality compared to conventional BH techniques. This FB technique can potentially improve the diagnosis and monitoring of NAFLD in children and infants.