What do you expect anyone to do? Gators and Crocs grab prey in seconds and then disappear into the water. The child was most likely dead within minutes either by drowning as the alligator dragged him under or by the massive trauma the wounds would have caused. Reading up on this, it happened at 9:30pm at night. Which means a dark and murky lagoon. So by the time the parents probably noticed the grab or noticed their child was no longer on the land near the lagoon it is too late.
Yeah, have some of you guys never seen footage of an alligator attacking something? They're pretty big (or can be) and also pretty fast. Add in that they often kill their prey by quickly pulling it underwater and it's not hard to imagine how the parents being maybe seven feet away and looking away for literally two seconds could have ended in this tragedy.