Not sure what kind of soil you're using, but if it came fertilized out of the bag, it may be a little too "hot" for your seedlings. I see some perlite mixed in, but not enough in my opinion. Every commercial potting soil I've ever run across needed to be cut with at lease some perlite. It loosens the soil up and makes for much more tolerable conditions for the roots. It's too late to worry about adding it now, but in the future, definitely use more when planting into any kind of potting container.

Just keep watering like normal, and hope for the best. Don't add any fertilizers, just water. Also, if there are still significant temperature differences between day/night, you might think about bringing them inside and growing them under a couple of CFLs, then transplanting outside when they're a little more hardy, and able to tolerate outdoor conditions a little better.