Birds are usually frightened of anything new. It's a natural behavior that's one of the things that keeps them alive longer in the wild. If they see you walking around of the sand, using the toe of your shoe or boot to move some of it around as an imitation of their scratching in it, they'll get used to it faster. After doing all that, I'd throw a treat they're familiar with and like on top of it, like scratch or sunflower seed. If that doesn't do it, you could also pick up a handful of sand and bring it over to them. Let them see it and peck at it. You could also dig around in it with your finger as they watch and then let them see it again.
It only takes a couple of minutes to do these things. Not only does it teach them about the sand, it teaches them that when they're frightened of something, they can depend on you to show them it's safe. I find that my birds get over being frightened of things right away when I show them it's safe, because they trust me and are used to me teaching them about things. Or, you can just leave them to figure it out on their own.
