OP, I think had there been several kids with her like you had when you were a kid that would have been okay. However, she was all alone. I personally would have stuck around until her parents got off the ride and tell them how glad you are to see them because you were JUST about to call the park security team for this vulnerable child. Thats just me, and I have done something similar to this and was glad I did, as a Mom left her 4 year old and infant in a stroller on a beach boardwalk while she got high in a condo nearby. And I called the police. It may sound normal to assume that you would stop in that instance, but my Sister, Sister in Law and many other people walked past without a thought. And then I got shamed for "butting in". Cops thanked me though, and the kid, who was freezing cold was wrapped in a blanket from my own stroller supply.

I dont think you are an asshole. I think its a matter of perspective. If you grew up with a pretty normal family, its not uncommon to assume others are normal too. Its just not always that way.