We took DD8-going-on-28 to WDW the first time when she was almost 3. She's been an average of twice per year since, so she's a Disney vet and a real DVCphile. I'll start by saying that any resort will be just fine.
As far as OKW is concerned, we own there and it's our favorite place to stay. We stayed at BCV a few years ago and it was very nice.
But SAB is not a great place for young kids who enjoy swimming, IMHO. I think it's a great place for little tykes who don't swim because of the sandy beach area (once you get past the duck poop in the water) and for adults who want to sun. But most of SAB is pretty much unusable for swimmers your grandkids age because there really isn't any place to swim except areas with strong currents.
Our girl is a very good swimmer (we were swimming in the Keys yesterday and SIL was complaining he couldn't keep up with her!) and we have a pool at home. When we were at BCV and headed back to SAB for the second time, she said, "I want to go to the pool with the sandcastle and lighthouse (OKW)!"
I also don't think THV would be a good choice unless the kids are outdoors types and would enjoy seeing critters. There is only one pool there and it's nothing special. At all.
Any resort would be fine, but to me the pools at OKW or SSR are much better for kids than most of the others (haven't visited the pool at AKV).
The buses at OKW are usually rated the best in the system, but you'll hear good and bad about buses everywhere.
Any resort would be fine, but one thing you MUST do with your princesses is the Princess character meal at Norway. They'll have a photo op with one princess and 4-5 others will come to your table and spend actual quality time with the girls, posing for pictures, signing autographs, etc, etc. Best character experience for a young girl at WDW, IMHO.
The suggestion of Bibbity Bobbity is also an excellent idea -- especially if combined with the Norway character meal. Bring money, because BBB costs an arm and a leg. Also plan plenty of time, because this level of beauty does not come quickly. Seriously, allow 2 or more hours.
The other thing I would suggest is not to try to do everything unless you are taking a long trip. Whatever you do, allow plenty of time for the girls to absorb and enjoy the experiences. Little kids are built close to the ground and they have a very different perspective than we do. They will see things through their own eyes, no matter how much we try to make them see the World through ours. They will find wonder in things we don't even notice or find mundane -- let them do that.
The other thing I would keep in mind is your role. Parents have different responsibilties -- a grandparent's job is to spoil the grandchild. Spoiling is both your right and your duty. Do it well.