Hi Shari!
My daughter absolutely loves Disney's Descendants! When we were there in February I looked everywhere for Descendants merchandise or characters and I couldn't find a thing. However, the merchandise and characters are always changing at Walt Disney World. I suggest asking a Disney Cast Member when you arrive to call the character hotline to check to see what characters are available at the park that day. You can also check the My Disney Experience app on your phone to see what characters are available.
The best place to check for merchandise is
The World of Disney in Disney Springs. They have the best selection of merchandise and would be your best bet to find the Descendants gear.
In regards to the best park for those ages, I think they will love all four Disney parks! If they like animals, they will love Animal Kingdom, My 9-year old daughter wants to be a vet when she grows up, so that is her favorite park. If they love movies, definitely check out Hollywood Studios. They will find Toy Story Mania, Rock n Roller Coaster and The Great Movie Ride. If they love to try different foods from around the world and like shopping, go to Epcot. Epcot also has some of the best attractions, including Test Track and Soarin'!
Magically,
Stacy