Been going to Kauai for 3 years now. First time stayed at Sheraton. Nice hotel, very good beach (ok not the Hyatt, but not the same $$ either). This year we stayed up in Princeville at a condo. Drove around the Westin property. Nice, kind of sterile. Westin's not full service but SVC. Think w/2 yo you'd be better off at Sheraton and drive over to see Hyatt. The last 2 trips the kids hung out at beach at Sheraton though we weren't staying there.
GOod luck