Three Pi's is good enough to be able to do something interesting. As B.Goode says, definitely check out the resources, especially the Physical Computing stuff.
The RPF have also published a Digital Making Curriculum, which may be worth a read through for some inspiration.https://www.raspberrypi.org/blog/digita ... urriculum/
If you are able to run some VNC Viewer software (like RealVNC, which is free and works well with the Pi) then you could build robots with your Pi's and 'set them free', coding/controlling them remotely from the PC.
Have a look in your local area (I'm assuming you're in the US?) for Raspberry Jams. These are get-togethers where you can go along and have a go at some Pi-related stuff, ask questions, get help and ideas. Also, if there is a Picademy (RPF's free CPD course for Digital Making) near you anytime, get on it!
I would imagine that for your target audience, then project or challenge-based working would be the way to go; something they can really get their teeth into from concept to completion.
I run computer clubs in a UK primary school and over the last couple of years, some of the projects that our kids (aged 8-11) have done are;
Various robots - hacking old educational bots, or old toys, or building from scratch. Using various control methods, such as VNC, bluetooth controller,
custom gui interface;
Interactive displays - using neopixels to represent traffic flow at a traffic-light-controlled junction, or for a Cycle Training display which highlights
the hazards for particular manoeuvres;
Smart Home - controlling heating and lighting with a tablet-based interface, or switching lights off automatically using sensors to detect human
I hope these will give you some ideas.
If you're looking at Python and the kids are new to it, you may like to have a look at Edublocks.org. This is a Scratch-like Python editor, developed (by a 12-year-old) to help students make the transition from block-based languages to text-based. It is downloadable for the Pi and incorporates some great libraries for Digital Making, including gpiozero, Minecraft, and Sonic Pi.
I hope some of that has helped. Let me know if you need any other info.