I agree with @leo - the synth makes a great intro to littleBits, and electronics in general. While I work at a college, where the kids average a bit older than your students, my daughter is in 9th grade, so I have a feel for where they are coming from.
Some of the kids will not be engaged with general electronic tinkering, making robots and other kinetic things will not appeal to a bunch of kids. With a 2:1 mix, it assumes that all the kids will be equally engaged.
All kids like to make noise however, and the synth kit excels at that!
Best of both worlds would be synth + other bits. So maybe sprinkle in a few synth kits with your steam sets.