Nothing yet. There are 50 law enforcement officials sweeping the area but to be honest, hopes of finding anything are pretty slim this long after the attack.
There's a lot of different versions of what happened. The Sheriff seems certain that the boy was wading alone in water about a foot deep, and the alligator wasn't spotted before it attacked. The dad tried to get his son but failed and alerted nearby people who got a lifeguard. The mother is also thought to have tried to help.
It looks like the signs will be changed to make it very clear that no one should enter the water at all. There's talk of getting hotels to include this info in the rooms but as it doesn't seem that the family were staying at the resort, that might not have mattered.
It's devastating for the them, but as there are no reports of recent sightings or any type of attack previously, I don't think WDW have been negligent.
US law is a lot more complicated then UK unless you trained there, but the American Red Cross suggests it's not legal to enter water with no swimming signs. "If an area of water has not been designated as a safe swimming zone, or has signs advising not to swim, always assume that the area is too dangerous for swimming. Entering water in these circumstances could be trespassing. (I've removed some info here about sharks). Alligetors, snapping turtles and snakes inhibit certain freshwater areas. Alligators are very dangerous, especially to small children, and can be found in freshwater in Florida, Louisiana, Georgia and Texas. Do not go into water that is not marked as safe in these areas. Do not enter water known to be inhibited by alligators. Stay away from the shore in these areas. If you encounter a snake, leave them alone and swim away slowly."
If the above is true, there's probably no legal liability. It seems WDW are doing their best, they are looking after the family and helping to try and find the boy.
Horrid situation all round and much scarier than anything we would encounter in the UK.