They do not chew too well, at this age.
Hence, lots of un-chewed food. A kid this age does not completely macerate their food. When chewing.
And some foods, like corn, are insoluble fiber.
Does not dissolve, in water.
"Insoluble Fiber: Insoluble dietary fiber doesn't dissolve in water and passes through your digestive system largely unchanged. It is estimated that 65 to 75% of dietary fiber in our diet is "insoluble."
Insoluble fiber may be found in bran (the outer covering of corn, oats, rice, wheat), whole grains (corn, barley, rice, wheat, oats), cereals, edible skins of fruits and vegetables, celery, brown rice, and some vegetables."
Here is the complete link to this article, about fiber and poop: