Wayne, you asked about safety. Dust mask and eye protection. Beyond that, 1/4" poplar - a simple pattern and let them at it. I found with my grandkids that if I tell them to think about the line on the pattern as "food" for the blade and their job is to keep feeding the line into the "mouth" of the blade. This helped them to recognize how to manipulate the pattern.
As for mastering corners, I drilled a pilot hole and had them practice turning the workpiece 360 degrees with slight pressure on the back of the blade. This helped them understand how sharp corners are made.
Bruce