Sunday School was provided to conscripted labor as a way to become literate on the only day that they had an opportunity. Sunday School was a community service provided by the Churches in Europe, and as an outreach was an effective way to also give children a basis in theology by learning to read from Bible stories. http://en.wikipedia.org/wiki/Sunday_school is an acceptable summary. Christianity Today adds to this: http://www.christianitytoday.com/ch/asktheexpert/whendidsundayschoolstart.html. Publicly funded schooling in partnership with child labor laws made this outreach take a turn towards the christian education.
To see this kind of Sunday School in action refer to the stories of DL Moody who spent great energy on recruiting street children into pews at the church that he rented in addition to inviting childrent to Sunday School in Chicago.
As an employee of a Sunday School publishing company I can speak to our intent. We provide underequipped and undertrained Sunday School teachers throughout Mexico and Latin America resources to teach with so that the teachers may put more energy and effort into working with the children.
As for the expectations placed on Sunday School teachers I think you may find that many churches only expect their Sunday School teachers to babysit the children so the important adults can be free from distration. I was told several times in one church that my goal was to make kids like church and if they learned some too, that would be ok. Many churches would not be so bold as to say it this way, however their children's ministries budget/volunteers reflect this mindset when compared to the rest of the church budget.
As a reference point consider the following:
Before a child reaches the age of 13 they have made up their mind on all major doctrines and will not change their mind on 90% of them in their entire lifetime.
Children are 5 times more likely to respond to a call to change than an adult.
Children usualy comprise about 30% of the church population.
These claims are pulled from Barna's work on Children's ministries today: http://www.amazon.com/Transforming-Children-Into-Spiritual-Champions/dp/0830732934
Teaching children the Bible in a way that makes them personally applicable, relevant, and meaningful to children is a need that is being met by the Sunday School publishing industry. Group Sunday School Publishing Board