To test this hypothesis I would select an even number of children (20 children, for example) and divide them into two groups. Each group would undergo a series of medical examinations to assess everyone's health (it is important that all children have a healthy body). Then each group would receive a "mission", the mission of group A would be to go to school only by bus for 6 months, the mission of group B would be to go to school just walking for their months. After this period of time, the children would be submitted to the same exams so that we could assess changes in the health of each one of them.
One of the most important principles of scientific research refers to the establishment of an experiment so that it is possible to collect data that can confirm or cancel a hypothesis. As we can see in the example above, this principle was applied, since a 6-month experiment was established, which will provide data, so that it is possible to confirm or deny the hypothesis that children who walk to school are healthier than those who go to school by bus.