@WillWorkForChocolate, as is so often the case, I’m sensing that there’s a question behind the question.
The real issue here is not whether the 7-year-old ought to have been left alone—I don’t believe you’re in any doubt about that—but the fact that these neighbors are taking advantage of you. And it goes way beyond manners, or the lack of them. They’re presuming upon your sense of responsibility by shirking theirs, and without even having the grace or decency to talk with you about it and ask if you’re willing to fill that role.
What’s more, it seems to me that there’s a potential liability there—that you’re assuming a risk of some kind by taking the part of the surrogate parent, without even having a formal understanding with their parents about it. What if you had to respond to a medical emergency with one of these kids, for example? Or what if you had to make a judgment call that the parents turned out to deeply disagree with?
I don’t like the position you’re in, neighborliness notwithstanding, and I don’t think it’s wise for you to let it continue without a frank discussion with the parents.
At the very least, they owe you the courtesy of asking your consent to use you as a free babysitting service.