Always stay within the law.
Even if it kills your child.
"It" hasn't killed anybody. Most Pits don't.
What alot of the "Kill all Pits" crowd advice being given here is a felony.
Chaining the dog to a stake in the middle of the yard is absolutely a recipe for disaster and will make ANY dog a serious threat.
Man up and deal with the owner. I'd ask him/her why they thought it was necessary or a good idea to chain a Pit to a stake in the middle of an empty lot.
I would certainly check to find out what, if any, laws the owner of the dog will be violating by chaining the dog to a stake. Lots of stuff that can make that "not humane". Be a huge pain in the butt about it who cares if they look at you first if something happens.
While I am not a member of the "kill all Pits" crowd, I am a "keep my family safe" member so if you still feel strongly that your family is at risk then by all means do what you believe is necessary to take away the threat. I would refrain from posting on an open forum about it though, because like I said earlier, like it or not just up and killing a dog on someone else property while it is chained to a stake will almost certainly get you jail time and use the wrong method to do it could lead you to lose a ton of rights that you now take for granted.
Let the bashing and insane posting of killer dog links begin!