Congrats!!! We were very similar this past April/May - stayed at the Poly, Garden View, with Dining, with our 2 and 4 year olds. Definitley a Great choice!! The convenience was wonderful with the little ones! I can give you some tips based on what we did:
If your kiddos still nap (ours both did - one of my TOP reasons for the Poly, just hop on the monorail!!), I'd highly recommend a 1st floor room. We had 1st floor Tokelau, and it worked well. We came back to the resort every day after lunch, went for a swim (they LOVED the Volcano pool, couldn't get 4 yr. old DS off the slide), then put them for a nap. DH and I would sit on the "patio", reading and relaxing. If we wanted sun, we looked out onto an open, grassy area, we just pulled the chairs out into the grass. We could take turns taking walks, going on a beach hammock, or getting snacks at Captain Cooks (btw, unless it changes for 2008, our fave uses for the snack credits were Dole Whips (DH) and chocolate chip crossaint (me) from CC's). The resort is sooooo gorgeous, you'll enjoy taking walks and soaking it all in!
I'd definitely recommend pacing yourself, see what the kids could handle. I had planned a fairly laid-back itinerary, and still ended up scaling back. That Volcano pool was such a HUGE hit, definitely made more time for that. We spent 3 "days" (opening or soon after until 1 or 2 pm) and 1 evening at MK, 1 "day" and 1 evening at Epcot, 1 "day" at AK, 1 "day" at MGM, and 1 "day" at Blizzard Beach. Oh, if your girls like the water, I'd HIGHLY recommend BB. The kids (ours wore Speedo vests) could go on everything but the speed slides. these were some BIG slides they went on! They have a great family raft slide, a double-tube slide, and other great single ones. TWO great kids areas, too. One for younger, one for older. We found that both kids enjoyed both!!
Hmm... what else?? One thing we loved about the Poly was being able to watch the Electrical Parade and Wishes from the beach/hill. When we went, Wishes was at 10 most nights. The parade is always at 9. We watched the parade, took the kids in, got PJ's on and then took them back out to watch Wishes in their PJ's, then it was right to bed. Worked great!
As far as meals, for that age I'd highly recommend Chef Mickey's and Crystal Palace - they LOVED both. We did brunch (late breakfast) at CM and dinner at CP. We also loved Kona - that was a great 1st night meal at the Poly. We arrived in the AM, checked in at the desk and went to MK. Came back when room was ready for naps, then went to Kona for dinner. Not going anywhere the first night was great, right after dinner, DH took the kids swimming while I got the room all settled. We also Loved LeCellier, it was fine for the kids. Really enjoyed Wolfgang Puck and thought Whispering Canyon was so-so. Your girls might love a princess meal!! I will likely do that just DD and I next trip. This time we just visited the Princesses in Toontown.
OK, well I'm writing a book!! sorry so long! Feel free to PM me if you have other questions about the Poly or traveling with kids that age anytime during your planning!! I'm jealous of you going next year, we have to wait to 2009 now!!