Children falling sick regularly.... prevention tips?

My 2.5 yr old just started playschool 2 months back and since then we have been having regular cough and cold bouts at home...My 9 month old has since been constantly getting the cough and cold from the elder one..This is the sixth time in a cycle we have been getting the sickness... the cough intensifies at night time when they go to bed. Tired of the allopathy and homeopathy medications given to them, since the cough seems to return 2 days after the course completion...Is there any way to prevent the cough and colds/ for kids to have better immunity? Also Im not sure what is causing the illness... is it the elder one or is it cause of some food or the air conditioning or the dusty weather outside ? Please help