Normally children get frequent trivial infections. It is extremely difficult to prevent all episodes. Some things you can try is:- 1.Have a well ventilated house. 2.Dont take them to Crowed places like cinema theatres , malls etc. 3. Avoid outside food. Give fresh home made food. 4. Use clean water for both drinking and cooking. In your case , 2 year old child can bring in infections from outside to the smaller twins. Avoid sending him to a play school for one more year.