Thanks everyone for your ideas!
My sister and her husband's kids are a 7 year old boy, who is not the bravest kid around, and a 4 year old girl, who, at this point, is more likely to want to go on a thrill ride then her brother. They are into Star Wars and most Disney movies. As a family they are not much for long (and expensive) trips, making this much more of a one-and-done trip for them.
I am the Disney nerd of the family, and we tend to travel more. I did drag my husband to Shanghai
Disneyland a few years ago, but that was only a half day trip. I could see a second trip sometime in our future, but that is not for sure. (Also, we of course don't know who my 3 month old will grow into.)
Without turning this into a sympathy thread, the trip is using money from my mother who very unexpectedly passed away. They day she died we happened to be discussing taking a family trip to Disney at some point in the future so that is why this trip became so important. While I want to make sure everyone has a great trip, I do want my little guy to have some form of memory, as this will be the last gift from his grandma.