That’s awesome! I just fed my outside batch with grow pack and they absolutely love it! You can start feeding them as early as four true leaves appear like mine 
And I got my CO2 in there as well in a small 500 mL bottle where I put 300 mL of water with 120g of sugar, 5g baking soda and 5g active yeast. Yes you can supplement CO2 this young and you will see the accelerate and growth like Superman LOL. It does help them grow faster.
Until the four true leaves appear I just feed them water with CalMag and gray white mixed together and pH to 6.3-6.5 for soil grow.
The paper towel you see in there is one of my thermometerometers covered from the light which makes it higher temperature if uncovered which is not a real temperature. Guess how I learned that
lol and lightly burned my seedlings. That was a long time ago but lesson learned! That usually happens with intense lights not cheap lights. When I was using Mars hydro, Roleadro Galaxyhydro, Atrium, and some other cheap Chinese crap, they don’t really affect the temperature on my thermometers because they are not that intense and they don’t reach them but once you step your game up and get better light like Platinum LED or BlackDogLED or Kind LED K3 or K5 series is what usually home growers get.