As long as the 16 year old and 11 year old stay together, I dont see why not. Assuming you know your children, and you know they are responsible enough to do this.
Just have very strict meeting times. And explain that if they arent there at that time they wont be allowed off again. If you all have cell phones, establish check-in times. Have them call at certain times and you can expect a call at certain times.
My parents let me take my brother around so my parents could have alone time and we never had any problems. My brother and I are 8 years apart. I felt so grown up being able to do that and I called my parents on the cell at certain times and had to be back (either to the hotel, or a designated meeting spot, at certain times) The Disney bus system makes this entirely possible.
I think all kids should experience this. When your 16, the last thing you want to do is hang out with your parents all day. (now i wish i could hang out with my mom all day! haha) So let them blow off some steam, they'll be appreciative of the time you do spend together. (even if its several years later)
Not to mention, assuming your kids are in public school...
In marching band in high school we went to theme parks all the time. at 13/14 your allowed to venture off with your friends without adults. But you have specific meeting times and you have to stick to them, OR you get stuck with the adults (who never ride anything) all day. We went to london as well, and ventured off alone.
Good luck!
