Hi Deborah,
I have an 8-year-old daughter, and I think that 7 and 8 years old is a great time to visit Walt Disney World. Kids at this age are able keep up with the energy needed at Walt Disney World, but they are still young enough to have a sense of wonder in their eyes.
I like you plan to visit one park per day. You know your children best, so I recommend that you think about which theme park you think they will like the best - and then save that one for the last day. If they are animal lovers, this might be Disney's Animal Kingdom. If they still enjoy the Disney Junior characters, it might be Disney's Hollywood Studios. If they like to explore countries from around the world, it could be Epcot. Many people recommend that you save Magic Kingdom for last, as it is classic Disney and the picture that many children have in their heads of Walt Disney World.
Before you go, it is fun to have your children make
customized maps of the Walt Disney World theme parks. In doing this, your children can choose which attractions they want to do the most. If you let each child choose three attractions per park, then you can make it your goal to hit those three attractions for each child. When you are at Walt Disney World, arrive at the theme parks early, even 30 minutes before the published opening time if you can. This way you will be ready to head into the theme park right when it opens, which tends to be the least busy time of day. Do as much as you can while everyone is happy, but when the twins get tired, be sure to take a break - maybe get some ice cream and just sit down and relax for a bit.
I would love to help you plan more Deborah; however, I am limited in the amount of space that I have to answer. Please do not hesitate to ask if you have any further questions!