For the Arduino, Scratch for Arduino (S4A) allows interfacing the projects together. A dedicated sketch is loaded on the Arduino which broadcasts and receives the necessary Scratch hooks over the serial port. This is a bit limited in that all the ports are hard-coded, but for a small project like three LEDs, it is fine.
python library written by Simplesi which allows communication between the Scratch hooks and the GPIO pins. Like S4A, the input and output ports are hard coded, but again, this is minor for a small project. Python is also easy enough to read to modify the library if needed.
Here are links to the three "Intro" sheets I had available depending on which board or language was used:
Overall, I thought this exhibit went well and met my goals of introducing a wide range of ages to the available methods of both popular boards. There was a great deal of interest from both the children and adults.
I wrote more about my overall experience at the Maker Festival on another post.