The first thought that comes to my mind is that you must have a very stable population if your congregation plans to do "outcomes" at the end of 6 years!

As you consider how to manage this, how will children who have erratic attendance be accommodated? Or students who have started coming to Sunday School in grade 5?
Who will write the test and what will be done with the results? Will some children "flunk" Sunday School and have to repeat certain years? Will the information be used just to tell leadership what has been learned and by which students over the course of the years? What will be done with the test results? What will motivate families to want to participate in the test? (or method of measuring outcomes?)
My thoughts are that you should think very carefully about the next step and how this will be communicated to children and their families. My concern is that there would be negative reaction that would not balance any positive feedback you might gain.
If your goal is to have an understanding of the effectiveness of your program, you can look at doing yearly evaluations by asking the children, their parents and the shepherds/workshop leaders key questions.
One way I like doing evaluations with the children is to play review games and make it really fun. It helps us know what the children have learned and retained in a way that is non-threatening.
Shepherds and workshop leaders often have a good sense of how things are going with certain workshops or even grade levels. What may have been a great workshop for the 4th grade was totally beyond the first grade. This is very useful information and gives you something to work with for future planning.
Having goals and objectives for each workshop, each unit, and for the entire scope and sequence makes good sense from an educational standpoint. Evaluation is important for the health of your program. Look at ways that make that possible in a way that will be positive for all concerned.
Then, perhaps we should all be doing outcomes for the adults in the congregation at the end of the year, based on their Bible knowledge and retention of sermons preached??

Jan Snell