Big Thunder Mountain Foot Incident: On March 10, 1998, a 5-year-old boy was seriously injured on Big Thunder Mountain Railroad when his foot became wedged between the passenger car's running board and the edge of the platform. All of the toes on his left foot required amputation and Disneyland made major safety improvements to the ride.
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.