Im currently grappling with this same question (we sail in May). I dont have any first-hand experience with either of these beaches, but Ive heard from friends that both of these beaches can have relatively high surfs (not suitable for kids). Ive heard that Cabbage Beach (near Atlantis) can get very high waves, while Cable Beach is slightly more placid. I suppose this depends greatly on the weather at the time. Another option you may want to consider is to take the Atlantis Beach Day excursion which gives you access to the Paradise Cove area at Atlantis. From what I understand, this cove is still part of the ocean but significantly protected from the high waves (and large sea animals if you know what I mean). As a result, it seems much more suited to a relaxing day at the beach (especially with young kids). Good luck!!