First of all, it's Disney's property so, yes, Disney is liable for what happens there.
Second, while it's common knowledge that there are gators in Florida, the family in question was not from Florida and may not have known what those of us in gator country have known all our lives.
Gators are cold blooded and, as a result, are seen mostly in the day time. So if you're from Nebraska, then you might not know that, although gators are seen mainly in the day time, that they feed primarily at night.
And being from Nebraska, they may have assumed that gators would not come up where there are people. Gators tend to be pretty oblivious to people and go where they want to go.
It's also possible that a previous guest may have fed the gator to attract it, changing its feeding habits and causing it to see people as a source of food.
There's so much more to consider than just "why weren't they watching their kid".