One of the most tragic cases did happen quite a long time ago, in 1977. Back then, the moat surrounding Cinderella’s Castle was much more accessible to the public and a 4-year-old boy drowned in it. The kid’s family sued Disney and won $1.5 million, in spite of the fact that the judge did declare that half the responsibility fell on them for leaving the child unsupervised.
There have also been many cast member accidents that resulted in death. Maybe this is not really the happiest place in the world to work in.