hmm tricky one

I would liken it to other rubbish people throw away and explain its too dangerous to pick that kind of rubbish up. That might answer the whats it doing there? people dont follow the rules properley and dont put rubbish in the bin
What were they doing with it? You could tell her some people use syringes for medicine if they are sick (true) but omit the drug related information until you think she is old enough.