The Short Version
While mechanistic research suggests high sugar intake can impair certain immune pathways, the specific claim that reducing sugar "significantly improves immune function in children" overstates the available evidence. The strongest studies in the evidence pool are based on mouse models or address chronic disease risk rather than measured immune outcomes in children. No pediatric clinical trial directly demonstrates that sugar reduction produces significant immune function improvement. The direction of effect is biologically plausible, but the claim's confident, child-specific framing is not substantiated.