I feel horrible for the people, but in all honesty, I'm a bit on the harsh side when it comes to things like this so I'll make it short and sweet.
First off, I'm sure there either had to be a dehydration issure or underlying condition. The sad part is, not everyone knows about underlying condition.
Second off, if anyone (like myself) thinks back to Mission: Space, I still hold to my belief that even though the boy met the height requirement, a 4 year old should not be allowed on that ride. For peat's sake he's not even in nursery school! Ugh. Bad parental decision, in my opinion, that will forever be second guessed. I feel horrible for the family - Disney won't ever be viewed as perfection/the happiest place on earth for them ever again.
Let's hope that the girl won't pass away like the boy did, that would just be awful. Totally awful.
Edit: To the poster who commented that deaths at amusement parks occur basically daily, you're right. Not deaths per se, but injuries as well. A few years ago a boy drown in the wave pool at a local amusement park near my house. There are people to blame for that - the lifeguard, for one, and parents, who should keep a better watch on a 10 year old boy. Many injuries are a result of fooling around while on a ride - and jumping out of a boat on Splash Mountain? Hello! Even though the water smells good I wouldn't put a finger in it! EW.
Your life is in your own hand when you step on rides at parks, it says so on the back of your tickets, just in a nicer way... It's the company's way of covering their behinds, which they have to do.