Problems with my seedlings

After having problems with germination I got a heated propagator. I was excited that all 10 seeds germinated...but after 4 days the 3rd and 4th leaves look as if been eaten by an insect (the first 2 are unaffected so far). Seedlings are 8-10 cm, I've prevented direct sun to them and haven't seen any insect on my room. Is it a fungus, too much water or humidity, too few natural light, the heater on the propagator? Is there any hope for them?

Thanks in advance.