Disneyland's First Death: In May of 1964, Disneyland had its first fatality. 15-year-old Mark Maples was injured and died three days after he stood up in the Matterhorn Bobsleds and fell out of the car. It was later reported that his restraint had been undone by his ride companion.
Seriously though I am surprised deaths don't happen more often. The Walt Disney parks had 132,549,000 visitors just in 2013. So just in that year alone the average was more than 37 thousand people per day.
I'll bet you it is statistically safer to play and have fun inside their parks than the trip flying or driving there.