We haven't tried OKW yet. My preference would be Boardwalk. The kiddie pool is just right for little kids and your 5year old might be adventurous enough for the clown slide.
We were at BCV in May for DD's 1st birthday. She did not like the pool. I don't know if it was the sand, the water, or the size of the pool. She pretty much clung to my DH the whole time. She has no problems at the BLT pool for the first half of our trip. DS (3.5 at the time) loved playing in the sand. But DS always asked to go back to AKV. He loves the animals and the water area (Kidani).
The only draw back for AKV is the longer bus rider to MK/Epcot/DHS. Having diaper bags, strollers, and other stuff is a pain in the butt.