PRotein Optimization With eXercise to Improve MUscle Mass and Functional outcomeS

This study tests whether an optimized protein and exercise program during a pediatric ICU stay can help children maintain muscle mass and recover better physical function after critical illness. **You may be eligible if...** - Your child is between 1 year (corrected age) and under 18 years old - Your child is admitted to a pediatric ICU (PICU) and requires a breathing machine (mechanical ventilation) within the first 72 hours of admission - Consent can be provided within 72 hours of starting mechanical ventilation **You may NOT be eligible if...** - Your child cannot receive tube feedings or nutrition through a vein (IV nutrition) - Your child has fulminant liver failure - Your child has severe kidney failure without dialysis - Your child is on ECMO (a heart-lung bypass machine) - Your child requires complete bed rest or continuous paralytic medications for medical or surgical reasons - Your child is at end-of-life or redirecting to comfort care Talk to your doctor to see if this trial is right for you.