It is important to take steps to strengthen your children's immune system to help prevent them from getting sick frequently. You can start by making sure they wash their hands regularly with soap, avoiding exposure to secondhand smoke, and cleaning objects they frequently touch when they are sick. In terms of supplements, while there are no miracle products, vitamin D found in multivitamins may help reduce the risk of respiratory viruses like the common cold. It is also suggested to consider giving your children probiotics, but it may be best to wait until more research confirms their effectiveness. Overall, focusing on good hygiene practices and potentially incorporating vitamin D and probiotics into their diet could help support their immune system.