It sounds as if your babies got "damping-off disease". You must always use clean soil (mix) when starting seeds or cuttings. You can buy "starting" mixes that are suppose to be safe, or you can make your own by baking it in the oven or the grill to kill pathogens.

Putting a thin layer of sand on top of the soil helps because it dries out faster than the soil below it, thus preventing the infection from starting.

For the plants that are surviving: try watering them from the bottom only to keep the top surface fairly dry. Good luck.