My 1-2" babies are dieing. What is the cause?

I had 4 NL growing and 3 Blue Mystic. However 2 of the blue mystic and 2 of the NL are now dead.
They have only been out of the soil and growing for a few days.
The stems on the dead babies are very then and cannot support the plant which is why they are dieing.
I dont think its stretching because I have a 600w light about 15 inches above them. Could it be that I overwatered the plants?

you could have but we need pics
do you have a fan on them to strengthen the stalk

Are the stems turning brown where they leave the soil? could be damping off from over watering.

It is possible that you over watered them. Too much destroys lol. Plus are you using nutes? If you use nutes too early on it can hurt seedlings. And what type of soil are you using.

You didn't say what medium you used. I know a common thing I see with people growing for the first time is they'll go to a hydro store, pick up a bag of Fox Farm Ocean Forest or Roots Organic and germinate the seed in that. What ends up happening is the seed gets burnt before or shortly after it pops up. Using any medium not specifically formulated for seed germination is risky.