Although it helps the plants to already have a bit of root (as Allen suggested), it's fine to transplant Flytraps when they are very tiny (as Matt mentioned), and like a lot of young, vigorous plants of many kinds, they don't suffer much transplant shock at that age. I usually transplant young Venus Flytraps soon after the first two leaves, the cotyledons, are fully extended, from their germination container into a more long-term container.
I usually plant a couple dozen or so into one 16-ounce styrofoam beverage cup with a hole punched in the bottom. If you plant them in a larger container like a 6-8 inch planter, it may be more difficult to transplant them later, when instead of algae to worry about, you have a dense mat of carpet moss growing, choking the roots and "bulbs" (leaf bases) of the tiny Venus Flytraps. It's easy to sterilize enough medium for several 16-ounce styrofoam cups, and just a few cups are likely to hold all of your Flytrap seedlings, unless you have many hundreds.
A moist toothpick helps in digging the plants up, poking holes in new medium, and transferring the tiny plants from one location to another, with a gentle push with a finger tip to detach the plantlet from the toothpick and settle it into the new hole.
The sand top dressing
seems like a good idea, but it is important to keep the top layer moist when the plants are so young, so you may have a little bit of a problem with that idea. For me, the best way to avoid algae (as much as possible) is to try to keep the medium moist but not wet, grow the plants in bright indirect sunlight for a couple months instead of direct sunlight, because in indirect light the surface dries more slowly and it's easier to keep them just moist without fear of drying out too quickly on the surface (which might kill the plants), and
never spray any liquid fertilizer or foliar feed anywhere in the vicinity. Even a tiny amount of overspray mist settling on the growing medium surface can induce a major growth of stifling algae. I learned this the hard way.
Steve