I second the idea of using pre-built robots if you are teaching a class of random kids, some who want to be there and some who don't. Our 7th grade science unit that I teach only lasts 3 weeks so we start with standard robots with two touch sensors and a light sensor. The students then spend the first week going though the
STEMcentric tutorial, getting checked off for each challenge they complete. Then we dive into a custom mission to Mars challenge that's roughly similar to an FLL mat but optimized to be more about software and less about mechanics. That's because we just have one class worth of robots but those are shared by four MS classes. Students can make mechanical changes to the robots if they like but they have to be simple enough that they can undo them by the end of the period.
The students grade for the unit is based on the score they get at the end on the mat.