Most likely the issue is overwatering. Those young plants don't have enough roots to uptake water quickly, and they really don't like having "wet feet". In veg, you want the plant to go through wet/dry cycles (let the pots dry out pretty well) to make the roots search for the water. To help the pots dry evenly when the plants are that small, you can sprinkle some water in about a 1" diameter around the base of the plant.