Yes, a layer of sand on top of the soil will prevent their eggs from hatching, it's just too dry and they rarely hatch, when they do, they're little maggots of sorts and the sand defiantly dries them out before they can escape it and find the soil.
It might be hard to put a layer of sand down that's deep enough to stay dry on top, it's just going to depend on the size of your seedlings.
Also, regular fly paper can help gather a few adult, probably not enough to hurt their population but you could at least I'd them properly, that might help treatment.
The most effective thing I could suggest would be a "no pest strip" (I think). It's made by hotshot brand and looks like a car deodorizer bar.
If you can contain the seedling in a small space with the no pest strip for a day or 2, it will almost for sure nuke any bugs that are in the enclosed area...
