My 9 YO DS is just now allowed to go within a block of our house without us.
I know Disney World is magical, and mostly safe. However, if a child predator wants a kid, DW is a very fertile feeding ground. My DS is pretty responsible too, but it's the grownups I don't trust.
As others have said, too, it's a family vacation. DH and I will split up in the parks, doing what each individual kid likes, but no kid of mine will be allowed alone in the parks until they are 14.
Of course, some day this might get me onto Oprah or Ricki Lake for being overprotective. LOL