alligators really aren't dangerous. gator attacks are extremely rare and most locals in louisiana let their kids swim in the river with them. i don't think disney really expected this to happen.
the alligator didnt even kill the kid. it dragged him into the water thinking he was something else, then let him go and he drowned. it's not like the alligator intentionally came out for human baby.
also, a sign is not warning enough at a hotel designed for children. many of them can't or wont read. they might not even NOTICE the sign.