My younger son, age 4, hasn't missed a class yet due to illness. But he also only has school 2 days a week!
My older son, almost 6 & in K, has missed 2 days. One day was the day of his Christmas concert - they said he could come for the evening program if he felt ok then, which he did. I was glad they were ok with it since he was really sad when I said he'd be missing school & his concert due to throwing up!