A Study to Learn About the Amount of the Study Medicine (Sisunatovir) in Blood and Its Safety in Infants and Children With Pneumonia Caused by RSV

The purpose of the study is to learn about the safety and amount of sisunatovir in the blood of infants and children up to age 60 months. These children have Lower Respiratory Tract Infection (LRTI) caused by Respiratory Syncytial Virus (RSV). LRTI is the infection to the lower airways such as lungs. This study will help inform the amount of sisunatovir to be used in future studies of sisunatovir in children. This study is seeking for participants who: * Are 1 day to less than or equal to 60 months of age * weigh more than or equal to 2.5 kilograms to less than or equal to 23 kilograms. * Have been tested to have RSV by medical tests. * show signs of LRTI. All participants in the study will receive many amounts of sisunatovir or placebo. Placebo is a pill that does not have any medicine in it. Up to 7 visits are required for the study. Some of these visits include checking participants health over the phone and/or a visit at home. The study will compare the experiences of infants and children receiving sisunatovir to identify the amount of sisunatovir to be used in future studies in infants and children.