You will have lots of fun at that age. What we did was schedule character meals quite often during the trip. That way kids were able to get autographs, we could take pictures with the characters and get a good solid, sit down meal. Now, when we go we try to do a character meal every other day. That lets us see most of the characters and get all the autographs. Be sure to buy an autograph book prior to the meal, though. I'm not sure if they will still be into this, at that age, but secretly I think our 16 year old neice really loved the photos and stuff...even though she shunned it. They are now in her room hanging in a prominent place.
Every time we've brought kids they have always wanted to do the rides multiple times. So, you might want to think about the best way to use fast pass.
The other thing to keep in mind is that, at that age, they may enjoy the water crafts that you can rent as well as the scurry bikes. It is something a little different that they won't be able to do back home.
As far as things to take. Before I had kids and took our neices, what surprised me the most was how many different sets of clothes they used during the week. Either they got wet from the rain and dirty from the food or playground. So, either plan to wash or bring a few more sets of clothes than you would normally anticipate. Also, try to have them bring two swimsuits and a pair of thongs for the pool. It is so much nicer when you can put on a dry suit instead of a wet one.
Also, if going in the summer, you will need lots of suntan lotion and make sure each one has a hat. Another summer thing is one of those electric fans. We each had one when we went in June and July and it was wonderful for standing in line. If they are snack eaters, it is cheaper to bring them with you or at least make a grocery stop. The little things really add up at Disney World.
Finally, make sure they have written down someplace your name, cell phone number, where you are staying and the room number. Make them carry it with them at all times. If a child gets lost then can become flustered very easily and it never hurts to have that handy for them to pull out. When you go into the park, be sure to point out to them who a CM is (by the tags) and tell them if they get lost or run into any problems they need to look for someone with a tag like this and get their help. At each park, you might also want to set up a "meeting place" where everyone would go, if someone was separated.
And, as far as souveniors go..both my neice's wanted to buy everything the first day. What we did was make the first three days or so mandatory "look" days. If they found something they really liked I would write it down on a piece of paper and then on the fourth day we would start buying. Often we found that they would have spent all their money at the start on things they ended up not really liking. Just an idea.
You will have an absolute blast. Enjoy yourself. This is a fun age.