The small rouded "leaves", known as cotyledons always go yellow and die, this is normal.
The "burning" on the lowest two leaves looks just like damage caused by the leaf tips hitting the soil, nothing more evil and not a concern.
At this age and if you are using quality soil, there should be no need to be feeding just yet, the soil should have enough nutrients for 3-4 weeks of growth easily and without the need for strong fertilizing.
Maybe half strength fertilizer once every 10-14 days until they have 8 sets of leaves would probably be more than enough, otherwise you risk causing damage to such a young and fragile plant.
Remember too, it is very, very easy to correct a slight under feeding, but very, very hard to rescue a burned and over fed plant!
Hope this helps, Organoman.