I would use earth rather than sand, or a mixture of the two. My daughter had a sandpit in the garden when we had our rough collie pup. He used to lie in the sandpit all the time, he just loved it but he started to get problems with weepy eyes and the vet reckoned the sand might be causing the problem by blocking the tear ducts, didn't cause any long term problems but a mixture might avoid any problems.
You realise of course that the dog will totally ignore it and proceed to dig your garden up