Ok, I can accept that your intentions were genuinely benign.
Your argument of "a small child within reach" is what's neither here nor there. Do you also warn customers not to let kids crawl on the tank because it may tip over on them? Believe it or not, this almost happened to me when I was a wee lad. I tipped my pop's tank over trying to get a better view. I suppose I could have died and (oh noes!) the guy at the LFS did not warn my dad that the tank could tip. Ok, enough of that. I'm trying to tone down the sarcasm.
Anyhow, to state it from the high brained side of things:
An owner of a LFS also accepts a responsibility to adequately care for the livestock they have. This means getting healthy livestock and only selling healthy creatures, giving them good conditions and doing all you can to increase their rate of survival once you sell them. IMO, you forgot about that part of the equation when you decided to kill them. I would indeed place a higher value on human life, but I would find a way around killing them. I still assert that it was the easiest solution for you.