I agree with the previous responses, don't teach the exact same lesson to all the grades. When I read your post, my thoughts were just what Marilyn stated. You can simplify some prep work by teaching a similar concept such as the elements, but change the project. If you were teaching shape to kind./first graders you may working on squares, retangles,circles,etc. where as the older grades are ready for organic vs geometric.

OR (like Rachel said) what about keeping the materials that you use each week the same for each grade and changing the concept: paint is used by all one week.