I notice that schools typically prefer to seat the students, youngest nearest to the stage and the oldest the furthest from the stage.

I challenge you to try something a little different. Since Kindergarten students are new at being an audience and may not know how to react, and in some cases may even become a bit overwhelmed by some assemblies (not mine); and since the older grades (especially 6-8 grade) may tend to be emotionally detached from an assembly (because they are“too cool”), try seating the older grades in the front rows on one side of the center aisle and the Kinder students in the front rows on the other side of the aisle, and the middle grades in the back of them.

Or alternatively, place the kindergarten in the entire second row. This may seem counter intuitive, but I suggest that you try it.

Be sure to tell the older grades that the kinder kids are watching them to see how they are supposed to react with audience manners such as attention, applause and so forth.

Additionally, I often have the pre-K students attend my shows. Do not put pre-K front and center. Put them to one side where their teacher can quickly remove a student if they become a distraction.