Hi, Corinne! It sounds like this is going to be a wonderful trip for you guys!
The way we do it in my family is I try to let each child plan a day. Fortunately for me, I have three kids who all prefer different parks - Disney's Hollywood Studios for the eight year old; EPCOT for the 17 year old; and Magic Kingdom or Animal Kingdom for the almost four year old. You can offer to let each child choose a day, then go through the website at
www.disneyworld.com to let them plan the day. Have them tell you what attractions are ones they don't want to miss, and let them choose the dining experiences they find most interesting. With six days, that leaves a couple of spare days for you and your brother to see and do the things that the kids haven't already planned, and it allows for time to revisit the theme parks that you've all had the best time visiting.
Book your meals as soon as you know which ones you want to go to so you avoid disappointment, and make sure you take advantage of the birthday offers for your birthday girl! If you are there on her birthday, she gets either free admission or one of the other wonderful birthday offers!
Happy planning!