I've removed sand stains from clothes using dawn dish detergent and some cool water.
I'm sure any other brands of dish detergent would work as well to get sand stains out of clothes but dawn dish detergent is what I used.
Mix 1 tablespoon of liquid dish soap with 2 cups of cool water and then sponge the sand stain with the solution and allow it to sit for a few minutes.
Then blot the sand stain until the liquid is absorbed and repeat the process for a few more times or until the sand stain comes out.
You can then place the clothes into the washing machine and wash as normal.
You can also add some baking soda and vinegar to the washing machine which will also help wash the sand stain out of the clothes.