I put my two-month old daughter in her car seat in the back of a 172 and flew for one hour to visit a gliding camp. She slept most of the way and then continued to do so during the day, with the tied-down aircraft gently rocking in the wind!
Her next flight was at four years old in the right seat of a Motor Falke (with lots of cushions in order to see out). Her main interest was looking at the "small cows".
I wouldn't take a small child flying if there is any hint of a cold.