We have increasing experience. So why should a Sunday School teacher who seems to us weak and simple and less experienced be called by inspiration to teach us?
One reason is that it requires humility on our part. It requires a humble heart to believe that you can be taught by someone who apparently knows a good deal less than you do, and perhaps seems less likely to get revelation. When I was the president of Ricks College years ago, I remember having a man who was my priesthood leader come to my house each month to interview me about my home teaching. He brought with him a notebook in which he wrote notes. He recorded not only my report as a home teacher, but my observations about the gospel and life as well.
I remember at first being very flattered. Then one Sunday he and I were visiting what was then called junior Sunday School. He was a few rows in front of me. The speaker was a little girl, no more than six or seven, probably not yet old enough to have the gift of the Holy Ghost. I glanced over at the man and noticed with surprise that he had that same notebook open. As the little girl spoke, he was writing with as much speed and intensity as he had in the study of my home. I learned a lesson from him that I haven't forgotten. He had faith that God could speak to him as clearly through a child as through the president of a college.