This trial tests a sugar supplement called 2'-fucosyllactose (2FL) to help children and young adults who have had a stem cell transplant. These patients often suffer from gut problems due to their treatment. The supplement is expected to increase good gut bacteria, reduce bad bacteria, and lower inflammation, leading to better overall health outcomes. 2'-Fucosyllactose (2'-FL) is a natural prebiotic found in human milk and is known for promoting beneficial gut bacteria and improving immune function.