I experienced a similar situation and decided from then on to get a second adult involved before assisting a stranger's kid.

In my situation, a girl was stuck at the top of the highest climbing fixture I've ever seen at a public playground. She was crying for a couple minutes without aid, so I walked over to tell her that I'd try to find her parent. The mother sprinted over and yelled to leave her daughter alone, then left the playground w/ her daughter.