Donald, you wondered why the bees would be more attracted to the pots with sand sprinkled on top than on the seedling pots that did not have sand. This could be because the sand layer, as thin as it may be, can still act as a "mulch" to shade the potting mix below from drying out faster than the other pots without sand on top. Since sand is a large-particle medium, the honeybees can push it apart easier to get down to the moisture below and fill up with water. This could be why you saw them "rolling" in it.
Regarding the shallow pans of water for your dogs, yes, the bees will use it, but it's dangerous for them since they could slip into the water, or get blown in by a breeze and then they'll drown. I have "portable" water-gardens up here in Michigan with my water lilies in them . . . an old bathtub here buried underground and a plastic "whiskey" barrel type in
the middle of our yard. The honeybees land on the lilypads all summer long and drink water from the edges. If you had a flat piece of lumber or other floating material in your dog dishes, you'd probably see honeybees using it as a landing-pad before they drank up some water.
Did you try putting some sugar water in one of the pots to see which one they liked better? ;-)
President - Mio Irisarians
Zone 4 (and a half . . .J)