My older two girls are teenagers, and I have a 6 yr old. Many times when we get to a park, we'll split up and either they go off on their own or DH goes with them, and my little one and I go off. Then we all meet up for lunch - and my kids never miss a meal so I know they'll be there! HaHa!
I am comfortable with them both being off together on their own, but I do make them stay at the same park as us.
About two years ago, we were staying at the Contemporary, and they caught the monorail and went to the GF to have the Villain Dinner - by themselves. DH and I, and my little one, went to Wilderness Lodge for dinner. My older two had a blast at the dinner (we had given them the room key to charge their meal), and they did a little shopping at the hotel, and were back in the room when we returned.