There's a fun new ABC store-called Country Store or something
Ko Olina has a new, fun store, sort of an upscale ABC Store, called Country Store or something (it's an ABC store) with a lot of everything-take out sandwiches, lots of beach stuff, souvenirs, etc. There are new eateries in that general vicinity also right by the ABC Store. It's across the street from the Disney Resort and very easy to find, especially if you're coming in from a flight. It's on the main entrance road before you arrive at Marriott's. The convenience and proximity factor is huge.
You might find that Kailua Beach is wonderful if you're up for the drive. Lanikai is right next to it and is considered the top beach in Hawaii. Waimea Beach up by Waimea Falls Park is a wonderful family friendly beach. Waimea Falls Park is a fun, educational stop for the family also. It's kind of like a nature preserve. And don't forget Waikiki Beach. It's famous all over the world and hard to top, especially since the kids are in school and you might have more elbow room.
The beaches on the Ewa side of the island where Ko Olina is located are pretty rough. Ko Olina was man-made by a Japanese development company back in the 80's and was bought out by Marriott's. It is not typical of that side of the island. Restriction of access to beaches is not allowed by Hawaiian law.