Is this nute or sun burn? What should I do to save my babies ?

That substrate looks very compact.. Your plant is starved off o2/co2. You can gently punch a few holes with a stick and then when it's fully hit the bottom, holding the top of the stick make small circles this will help bring o2 into your sub. As with low o2 and drowning n water with compact sub that's exactly what that looks like.

Hello my friend, a few problems here, you keep your soil to wet so the roots lack oxygen. Let them dry out in between waterings for the right water : oxygen ratio. And the nutrients inside the soil are too strong what caused a nutrient lockout for the small plants. To solve this problem you want to do a small flush and let it dry out conpletely to solve both of the above mentioned issues. Give the pot volume in water with pH 6.2 and without nutrients. Let it dry ou, you want the top layer (top 5 cm / 2 inches) of the soil to be completely dry. After this you can continue watering with small doses of nutrients. Stay below EC 1.2 during this phase and pH between 6 and 6.2 Give water till runoff (10% from the given amount) and let the top 2.5 cm (1 inch) dry out every time in between waterings. This should solve your problems! Merry Christmas buddy! 💚

Looks less like burning from the lights and more from the nutrients or the medium. If it was from the lights it would be the top most growth vs the lower. Have you followed any advice from your previous question about coir and being depleted of nutrients? Or to let your nutrients ph drift and then readjust? Are you monitoring your EC of the feeding solution? Its good to know whats going in to help trouble shot issues like this. If the EC is high then thats the issue, i have a feeling it might also be your watering habits n the coir used. Don’t forget to select an answer to your previous question to close them. Send a DM if your still confused and need help. Best of Luck!