I can see that your plants need some chemistry. Such as Nitrogen. I would take these plants away from the windows at night, even cover them with newspaper (excellent insulation and I am not kidding), only allow them on this window sill when YOU are there and the temperatures are at least 60 plus.
Your problem or rather your plant's problem other than too cold is fertilizer. It is clear to me your plants don't have enough nitrogen and to challenge them further with too cold? Ugh.
Please tell us more information. Even leaving these potted plants where they are but covering them with newspaper for night time will help. Stability and constancy is critical for plant's health and immune system. Once, sure, after too cold temps, plants will be able to regroup. Without the necessary chemistry plants need for photosynthesis no plant will be able to sustain an environmental accident or two.
Your plants need a balanced extended release fertilizer right now; Osmocote 14-14-14 extended release. And newspaper just loosely bunched up between window and plants. A sheet or two to cover the plants and even tuck in under the tray. Throw that one dead baby out into your compost pile.
Up pot the rest of your baby trees in potting soil in 4" pots, now. (Looks like 3" pots now, yes? Great size to start seeds of this little tree). Don't water so much. Allow the soil to dry and the pot should feel light when you lift it before watering again. I would also use distilled water or water from a private well. Up pot again when you see roots going through the bottom of the 4" pots into 6" pots. Use only potting soil. I kid you not. Are you going to be planting outside someday in the garden?
All you will have to do is acclimate these starts or baby trees to the out of doors. Never just plop your little trees that you are growing inside, out side in the sun. They will fry, sunburn, truly. The epidermis is very thin on plants grown in doors. To acclimate and thicken the epidermis for sunlight you take outside for 5 minutes a day for 3 days. Increase the time by twice for the next 3 days and so on until they are able to withstand at least 4 hours with no harm. Always protect with newspaper at night until they are completely acclimated to the out of doors. A covered porch really helps make this process easier. Oh, if plants out of doors need to come inside, this process is the same and just as crucial.
Do you have room to add a REAL grow light? That provides a bit of heat stabilizing the drafts and temperature changes. You've got a ways to go before these trees can be planted out of doors this spring? Is this a south facing window? Sure make great gifts...