This study tests a new treatment to help with droopy eyelids (ptosis) and eye movement problems (squint) in children and young people with genetically confirmed mitochondrial conditions - using red-light. We use a torch to shine a red light through the closed eyelid for 3 minutes a day. The study will run for 18 months. We believe that this treatment could help strengthen the muscle in the eyelid to make it easier to open the eyes, and could also help some children with squint.