The first two leaves will always die and fall off quickly. They have served their purpose to the plant by then. Half strength
nutrients for plants that size is plenty. Humidity could be higher though. Especially for seedlings. Try using a humidity dome to hold in humidity. 70%-80% humility would be much better for seedlings. Just make sure air temp under the dome doesn't get to high or you will have another problem.
P.S.
I'm sure you already know, but that lighting will quickly become inadequate as the plants get bigger.