We don't really know what happened, there's not enough information out to make a judgement like that.
So far, with the information that has been released, it does not look like the parents weren't paying attention at all as like PP mentioned, the father tried to pry the child away from the alligator. He would have to be very close to the child to get there in time to do that.
This is such a heartbreaking situation and I agree that it could happen to anyone. It's the type of thing that happens in a split second. I can't even imagine how the parents feel now, and the guilt that they must be feeling.
And even though I know there are alligators in Florida, and I have never even touched Florida water, ever, I have walked close to the water at least once (in many places where it is allowed to do so) and up until now, never considered that an alligator could just appear out of nowhere and attack someone. That was not something that was on my mind at all and I see now that it is a very real possibility. Even if you don't see anything in the water, there could be something lurking there.
Also, even though there are warnings near the "beaches", I always thought it was due to the water not being clean enough for swimming, not alligators. If alligators are a real possibility in those beaches, then there should be a clear sign about that very thing. Either this is not common or Disney needs to be more clear with their warnings when it's something as serious as this. I agree that it is a warning and I'll be extra vigilant if I ever visit Florida again.
It's such a sad situation and I'm hoping for a miracle even though it is unlikely. I'm also hoping Disney is truly supporting the parents and not just giving them a quick apology and some "compensation" and then ignoring the incident completely as they seem to do sometimes.