Amy, I was going to make the exact recipe for my art class last year, but had to change it because the concrete was toxic, and we aren't allowed to use it.
Instead, we made a "safer" version, by using plaster of paris. We used plastic plates as our tray, and then added gem-stones, rocks, sticks, etc. The kids loved making them, and were thrilled the next week when we popped them out of their trays!
Always do what you want, and say what you feel, because those who mind don't matter, and those who matter, don't mind