I'd pick Cayman. Stay on Seven Mile Beach. It's 5.5 miles of white sand beach, in most places it's also really nice offshore. There is some ironshore in a few areas, mostly at the south end so ask about it before you book your hotel. A lot of people book condos instead, they're mostly right on the beach. The diving is as good as Bonaire, maybe better. The vis certainly is. And you'll see more pelagics and bigger stuff there. Diveops will pick you up on SMB also with their vans. And there's the Boatswains Beach/Turtle Farm complex and the Butterfly farm for the kids to see.
Beaches on Bonaire are few and far between since it mostly sits on ironshore. There's a good beach at the Plaza Resort which is on the 18Palms divesite. But it's kind of spread out so a long walk between the divesite, the beach and their dive operation located on the lagoon at the south end of the property. Divi Flamingo has a strip of sand on their property, the casino, and you can walk to town from there.
Harbour Village has a nice beach area but not so great diving. Eden Beach has a decent piece of sand with a few palm trees and is on the Front Porch divesite. On the other side of the island, Lac Bay is a good beach but it's the windsurfers beach also. And the diving from there is back on the west coast about 10mins. away. Nothing in Bonaire is too far away though.
You might also consider Curacao, there's some good beaches there. Especially at some of the resorts in town. The Curacao Marriott comes to mind, it's a very nice Marriott with a nice beach adjacent to a nice pool area. The diving is almost identical to Bonaire except the reef is a little farther out in some areas and the drive between shoredive sites is considerably longer.
For day trips you could all go out to Sunset Waters, they have a nice beach area with a protected cove that the kids (with boots on) could wade in and snorkel while you dive there. There's also a bigger beach area on their property but we didn't go in the water there so I don't know whether it's clear or not - some of the "beaches" on Curacao have ironshore/coral rubble just offshore. Porto Marie is another beach complex with improved facilities, a snackbar and a good beach with diving just offshore. Curacao itself is a little rougher than Bonaire, more crime, bigger city etc.