It's not Maui, but my family was amazingly content on our 4-hour vacation at Richmond's Keller Beach. Unfortunately I took my photos with a Blackberry, so they look a little off, but you can get the idea of the space.
Plenty of wet sand area for building castles, digging ditches, and playing catch.
Plenty of dry sand area for lounging and dispensing snacks.
The water is calm, albeit freezing, which didn't stop my 4-year old from getting wet up to her waist and laughing maniacally.
The beach is adjacent to a large regional park called Miller/Knox. If you've ever been through the tunnel near the Richmond public indoor swimming pool, you've been right there. To access the beach, park on Dornan Drive and walk down the stairs near the Keller Beach sign. Make use of the restrooms halfway down the stairs to avoid a trip back to them.
How to get there for those unfamiliar with Richmond:
From I-580 West in Richmond, exit at Canal Boulevard. Turn left at the light onto Canal. Turn right at the light onto W. Cutting Bl., then left at the stop sign onto Garrard Bl. Proceed through the auto tunnel; Garrard becomes Dornan Drive on the south side of the auto tunnel. The park entrance will be on the right less than 1/2 mile south of the tunnel.
Bring a picnic — there is no nearby source for food.