Definitely not the smallest deluxe. Smallest is Wilderness Lodge and Animal Kingdom Lodge (both at 344 sq ft), increasing in size to Boardwalk (371 sp ft), then Yacht/Beach Club (381 sq ft), then Contemporary (394 sq ft), then Polynesian (415 sq ft) and then Grand Floridian (440 sq ft). All averages, of course, and there are some differences between rooms in each resort. If you've ever stayed in WL or AKL, you'll definitely feel the size difference of 50 sq ft between those and the Contemporary, whereas the difference between CR and Poly is only about 20 sq ft.
I'd say that the pool at Poly is better than CR, mainly because there is absolutely no shade at all. It's just a concrete hole in the middle of an open area. Unless you're talking about BLT, in which case I'd say that the pool is definitely better, as is the play area.
CR is better for location, IMO. While the walk may be an issue for a 5 and 7-year-old, especially after a long day in the parks, it's also no picnic standing in line at the monorail for a potentially substantial period of time. At least with walking, you feel like you're making some kind of progress, and you have the monorail as an option. The only options for MK to Poly are monorail or boat or walking to TTC to take monorail or ferry, which means lots of standing and waiting. The other benefit to CR is that it's a relatively short walk to the bus station at MK, which has buses to all of the other resorts, whereas the buses at CR or Poly only go to the parks and Disney Springs. That can give you a lot of options for dining or visiting other resorts more quickly. CR is not my favourite resort by a long shot, but there are some definite advantages to it location-wise. Plus Steakhouse 71 and California Grill.