I can't believe I had to scroll down so far to get to this. This sub has a really odd mentality of "you get what you deserve" and that's it. No nuance. And yes, that woman totally deserved what she got - but that poor kid didn't. I agree OP was in no way obligated to look after the child, but IMO he should have gone inside and informed the store staff what had happened before leaving, or called the police.

OP, I'm not trying to be horrible, you're young, she put you in an awful situation, and it sounds like you panicked. He was in all likelihood absolutely fine. But if anything like this ever happens again, I'd really urge you to try and get someone in authority involved, for the sake of that kid, who was blameless in all this.