I also think you should look into the Family Magic Tour. I did this with 4 Newbies a couple of years ago, and it was so much fun and really added to the MK experience for everyone. You should however, do it after they have visited MK once during your visit. This way they are not distracted at all the magic, but can find things they may want to revisit after the tour. What I did was not tell anyone what we were doing Just that we had to be at MK City Hall at 10am one morning for a surprise. We had a great time and finished with lunch at Cindy's castle. It was the best $$$ we spent on that trip.
Also, encourage them to bring pennies in a mini M&M's tube, and purchase a pressed penny book upon arrival. My kids always have fun collecting pressed pennies. Canadian pennies work fine, but you'll need 50 cents US for each. Pressed quarters are $1.25
Autographs are fun, but not sure 9 and 11 yr olds will be totally into it. Maybe you could purchase a white T-shirt from TDS or ball cap and have the characters sign them. That might be more fun since they are older. Just make sure you bring a "sharpie" permanent market. Remember, they cannot be wearing the shirt to be signed.
Also a fireworks cruise is also fun and gives you that feeling that you are viewing away from the crowds and special.
The comment from our Newbie was that they didn't realise there were so many "extra" things you can do besides the parks/rides, which is why I was glad we enhanced the experience with some of the other things you can do.