Honolulu-
-Kids love Hilton Hawaiian Village. Lots of pools and right on the beach. Agree that Ali'i Tower is preferable, with less expensive options available.
-Kahala Hotel is really nice, but very expensive, compared to HHV.

Hilo-
-Shipman Inn not a great spot for kids. Maybe Volcano House, which is located within the National Park. I would consider 1 or 2 nights Hilo/Volcano side instead of 3. Tack extra day(s) onto Kona/Kohala side where you will be at beach resort, and may get 5th night free rate.

Kona/Kohala-
-Fairmont Orchid is nice, but no real beach/no waves. There is a man-made cove with sand, but no comparison to Mauna Kea or Hapuna Beach.
-Mauna Kea Hotel is really nice with great beach, but it is not exactly kid friendly. They have a 3/per room policy, which you may be able to overcome with $75 rollaway bed, depending on kids' ages. (Check with hotel)
-Hapuna Beach Hotel is sister property of Mauna Kea, located right next door, and sits on what many say is the finest beach on the Big Island. It is a little less expensive than Mauna Kea, allows 4/room, and is very kid friendly.
Also, has a great pool, which you can walk back and forth to from the beach.