It seems like a tragic accident. Nobody's fault - just circumstances and a pretty rare incident.
Alligators, although common in Florida, rarely attack humans.
I have been to WDW several times and visited a few hotels on site, including this one. When we have STAYED at a Disney resort we were told not to go in the waters, only in the pools. We have seen alligators near resorts.
And we knew about alligators - the guidebooks mention them a lot, and there are warnings about alligators in many places in Florida and there are some in WDW resorts. I wouldn't go in any lake/pond/river in Florida - you see alligators in many places and it isn't uncommon to see them, they are even known to find their way into villa pools!
However, it appears on this part of the beach there was only a No Swimming one rather than a Don't go in the water/Alligators type one. So, this family probably didn't really realise the risks - think they'd only just begun their visit too - and it was just a tragic accident.
You can't prevent the alligators entirely, and although there were signs in other areas, for some reason at this place there wasn't.
Sadly I think it is just an accident with no one really at fault at all, sad as it is.
I am, however, surprised there was no one else around at the time - it was not that long off firework time and was potentially movie night time. Were they at a different part of the resort/beach and not at the main area?