If this is your granddaughter's first WDW I would definitely recommend a MK resort and, if budget allows, consider the Poly or GF. The Poly is quite magical, IMHO. The theming is magnificent and the view of MK and the MK fireworks from the beach is spectacular. GF is also gorgeous and if your granddaughter is into princesses, it is the most "princessy" themed IMHO. They have wonderful dining options for young children - 1900 Park Fare character dining for both breakfast and dinner plus a very special children's tea. They are also opening a Bibbidi Bobbidi Boutique in the resort this upcoming summer. Both the Poly and GF have wonderful splash pad areas for smaller children - Polynesian themed at Poly and Alice in Wonderland themed at GF. If budget does not allow either of these options, I would also consider WL. Again, a stunning resort with great dining options for young children - Whispering Canyon is a lot of fun and the new Storybook Dining offers dinner with Snow White and other characters. They also have a splash pad for younger children. If you stay at WL, it's also quite easy to walk or boat over to Ft. W for Hoop Dee Doo Revue (which is a ton of fun), the WDW stables to see the horses and go on pony rides, Chip and Dale's sing-a-long, plus a playground.
Edited to add: If you are planning a trip in September 2019 and then another 8 months later in May 2020, I would definitely think about how many days you will spend in the parks on both trips and then do the math to see if separate tickets or AP's would be more cost effective. As PP mentioned, along with the AP, you also receive Memory Maker plus dining, merchandise and resort discounts.