In the Mah Nishtanah at the Pesach Seder one of the questions asked is why on this night we eat only matzah whereas on other nights we eat both chametz and matzah. The question about this question is when, exactly, do we eat chametz and matzah together? Was this so common that a child would see the lack of chametz at the Seder as unusual and be moved to ask about it?
In the times of the Beis Hamikdash when people regularly brought korbanos they were used to following the regulations regarding the consumption of sanctified food. All those partaking of the food had to be tahor and the food had to be eaten within a certain period of time. Now, for most korbanos, the amount of food that had to be eaten, and the time limit for eating it, was quite manageable. There was one exception, however, and that was the Todah [thanksgiving] offering. This offering was accompanied by 40 loaves, 30 of which were unleavened (matzah) and 10 of which were leavened (chametz) and all of these had to be eaten in one day! The Torah purposely made it difficult for the person bringing a Todah to eat it all himself, forcing him to invite many friends and family members to the meal to help him consume all of the sanctified food in time. During the meal he would share the story of why he was bringing a Todah offering and this would lead to a sanctification of God's Name since so many people would recognize the kindness that God showed this individual.
Now, a child at the Seder would see a large group of people gathered for the consumption of an offering (the Korban Pesach, in this case) but would notice that only matzah was being served. He would find this odd, since at all the other large gatherings he had attended throughout the year when offerings were being consumed (namely, when his family was invited to someone's thanksgiving meal) there had been both chametz and matzah on the table at the same time. Hence the question.
(I do not have a direct source for this. Someone just told it to me in shul this Shabbos. Any ideas?)