I would say there's a significant difference between the chemical/biological danger from touching a material which appears basically identical to other safe mat

I would say there's a significant difference between the chemical/biological danger from touching a material which appears basically identical to other safe materials and causes toxic exposure over time, and the physical danger from falling or being cut. I want kids to learn that concrete is hard, and the harder you hit it, the more it will hurt. It's an easy lesson to learn, by trial-and-error, and transferable to many other situations. I don't want to make kids try to learn about lead poisoning by trial-and-error. They're not going to succeed at that. Even adults took 50 years of research to stop putting it in gasoline.