Ensure baby sleeps well. Breastfeed if you can (baby gets antibodies from you to fight infections). For older children, also ensure enough rest. Ensure good nutrition, including fruits and vegetables for natural vitamins and minerals. Supplements you can consider include vitamin C, probiotics (yoghurt/yoghurt drinks are ok too). Ensure you and your children observe good personal hygiene, especially washing hands with soap (7 steps of hand washing)
Good diet