When a business invites someone onto its property to conduct commerce, that business has the
highest legal obligation to make sure that its facilities are as safe as possible.
Disney does not simply have a legal duty to warn people. It also has a legal duty to protect them from all knowable dangers. Does Disney know that it has alligators within feet of where people walk, with no protective barrier between them? Of course. As
@Incomudro pointed out, this is a white sands beach with beach chairs and a movie night. Disney is inviting its Guests to this shore. Disney knows alligators exist in the Seven Seas Lagoon. A posted "No swimming" sign in no way reduces Disney's obligation not only to warn but also to protect. Legally, Disney doesn’t stand a chance on this one.
Beyond the law, Disney wants to show compassion for this horrific tragedy. Disney does not want to litigate this. Disney almost certainly will settle long before this ever sees the inside of a courtroom.