Jennifer, as Ken has stated. Leaving them in those cells longer will be best. Usually a good rule of thumb is, transplant up once seedlings have at least 4-6 true leaves. If you had them all in a communal pot, then transplanting might be sooner. This is not the case of your seedlings.
Definitely do not add more soil and cover the stems.
With these seedlings, I would not let them dry right back. Seedlings should be kept evenly moist, but not wet. Making sure that they are in a warm situation and good light.
They are looking really nice, Jennifer.