Thanks for your reply!!
I can tell you do have real world experience with kids this young!!
Yeah after the first 20 mins we realized the kids were wildly just snapping things together. The snap itself seems addictive to them lol.
So for the last half hour of our class we made them put everything back, gave them a focused task (light controlled by a switch), and then they were able to complete the task and actually answer questions about what function the switch is doing.
So for each of the next few classes we are planning to do the same, introduce just 1/2 new pink bits, and one new green bit, and ask a focused question (e.g. make a door bell, and choose the appropriate input bits so it's not too loud, and it turns off after u let go off ur finger).
Securing the USB down: ahhh masking tape. I was wondering about that (strong enough etc). Since it works for you I'll try that! I've also ordered some "flat" USB cables which should be easier to secure down and also eliminate the "twist" with regular circular USB cables that makes the circuit not stay flat.
In addition, as you've suggested, we will also keep some 9V around for the more mobile projects like cars/wearables etc, or when they come to the front of the class to show off their work