I had a great time dumpster diving at the university in the 70s. My dad freaked out when he realized it was a dumpster from the Microbiology lab. We'd been playing waterguns with surplus plastic syringes.

Although the risk is tiny, I would observe that very old plaster from walls has horse hair. There's a non-zero Anthrax risk, albiet very low. If a house is being refurbished then you may be playing in this, or in Fibrous cement (asbestos)

Kids need to learn to take risks. It's mostly a question of what risks we want to encourage and what risks we can wear.

"don't be stupid" covers most of it (c/f syringes above)

I learnt that and never stopped. 49 years of dumpster diving experience here! Dumpster diver unite!!!!