Feeling like a failure

Emily and Damien are allowed to play outside by themselves. We live in a fairly nice complex of townhomes, where there are lots of kids. They know all the rules:
-no going beyond the boundary trees (keeps them away from the street)
-no going in someone else's house unless you ask us first
-respect other people's property (no tearing up bushes, throwing rocks at cars, kicking cats, etc..)
-don't talk to strangers

Somehow I forgot to tell them what to do if they find some junkie's garbage. My 6 year old found a syringe on Saturday. Since Damien is the epitome of curiosity, he took the cap off and poked himself in the finger with it. I Thank god he did not just throw it back down and go about his business, but brought it to me, and told me what happened. I've spent my fair share of the weekend sick with worry, complete with bouts of self-accusation and bawling (in the shower mostly so as not to worry the kiddos). The syringe is now on it's way to a lab for testing, and Damien has an appointment this afternoon with his doctor to do whatever we can for him. I pray that this one moment of childhood curiosity does not alter his life forever.