I am no genius for growing cacti/succulents from seed but I don't like using that real fine sand at all. It crusts over and that occurance seems to wipe out the seeds/seedlings I've used it on. I've tried planting on the sand, in the sand, under the sand and adding sand a short while after germination.
What I've used to successfully germinate seeds is Shultz Aquatic Plant Soil a fine baked clay grit (buy in the Pond Dept at Home Depot or Lowes). Soak it well, let drip "dry" and then surface sow the seed, mist the surface and stick the wet pot into a ziplock baggie and seal it up, put in a lighted but not direct sun area and wait.
Once they germinate let them stay in the baggie for a while but open it every so often to exchange the gasses in the bag. Some months after germination you can start not zipping the bag totally closed and this will help harden off your seedlings into the world of dry air. If you just expose them 99% will die possibly all but if you slowly have the bag more and more open you'll be fine with minimal losses. Seperate the seedlings and repot after a year or so, depending on how fast the plants you have are growing. Using Shults APS will be a great benefit to you in this task as the grit will fall right off of the roots since it doesn't compact like soil does.
Against what people told me I even germinated a bunch of Echinocerus/Trichocerus seeds on plain old Hoffmans seedling mix, they did just fine but they are a quite hardy type of cactus compared to the Lithops & their relatives.