I ran into this same problem from September-January. My daughter was sick every 3 weeks or so from daycare. We went to a naturopathic doctor who recommended a probiotic:
http://www.naturamart.com/seroyal-genestra-hmf-powder-60g...
1/8 teaspoon after dinner for 1 week and then 1/8 teaspoon after breakfast and dinner for 1 week. You can repeat this after any course of antibiotics. She also recommended we do it for 2 weeks every 3 months. 70% of our immune systems are in our digestive tract because it is the first defense against all germs and if it is unbalanced, it is more likely we will get sick.
She also gave us a multi-vitamin and immune system builder and recommended taking them 2 x a day for a month, then continue the vitamin daily and use the immune builder when you see signs of a cold or sickness coming on.
They are from Mountain Peak Nutritionals and they are called:
Children's Multivitamin and Children's Immune Support. You can only get them from licensed naturopaths. You can call the company and ask them where you can purchase them:
1-877-MTN-PEAK (877-686-7325)
Since we have been following this regimen, my daughter has only had a mild stuffy nose and has had no allergies.
Hope this helps.