Thanks to medical advances, survival rates \>5 years in children/adolescents undergoing hematopoietic stem cell transplant (HSCT) exceed 70%. However, these patients are at high risk of suffering sequelae associated with the underlying disease and/or the HSCT itself, which negatively affects their physical capacity. These patients also tend to spend too much time inactive, which further accelerates functional decline in addition to producing fatigue and impairing quality of life. Therefore, new strategies are needed to minimize the morbidity associated with HSCT. In this effect, although physical exercise represents an interesting adjuvant treatment option for HSCT, scientific evidence in this area is still scarce. Implementation of physical exercise intervention in pediatric HSCT units is challenging due to the lack of research on the effectiveness, affordability and accessibility of this type of intervention. Therefore, establishing the effectiveness of physical exercise under controlled conditions is an important step. The investigators therefore aim to assess the impact of a physical exercise and health counseling program, compared to health counseling only (control group), in pediatric patients with cancer undergoing HSCT on physical-functional, behavioral, psycho-cognitive and clinical outcomes, and blood biomarkers. The investigators will also determine to what extent the benefits of this intervention are maintained over time. Additionally, the investigators will determine the feasibility of implementing the proposed intervention in a real clinical situation in 3 different pediatric HSCT units.