Hi Bill! What a wonderful group you have coming in May! I don't know who will enjoy it more the grandparents or the youngest child!
My suggestion for accommodations would be to consider the Yacht and Beach Club resorts as well as the Boardwalk resort. My family has been staying in this area for many years because we love the accommodations and also the location. You can walk to the side entrance of EPCOT at the International Gateway which is very convenient and has the same hours as the front entrance to the park. You can also take a water taxi from any of these resorts to EPCOT. The bonus is that you can also take these water taxis to the Hollywood Studios which means you can get to two parks without using the bus system. Busses conveniently take you to the other two parks and Downtown Disney Marketplace.
There are suites available at all three resorts which could accommodate your family. There are also the Beach Club Villas and the Boardwalk Villas which offer the opportunity to have some kitchen facilities. These are Vacation Club Villas and are great for the guest who needs something for a large family group such as yours. They are available when not occupied by members.
This is a great area for both adults and children. The pool at the Yacht and Beach Club is probably the best on property. You are close to EPCOT and all the fine dining choices you have in that park and you are right at the Boardwalk area that includes clubs such as Jelly Rolls and ESPN and other great restaurants!
Compare the amenities of these resorts at
www.disneyworld.com . Call central reservations at 407-W-DISNEY (407-934-7639) to see the availability for the dates you have chosen.