Yes, sounds like they are either wind or sun burned or both....unfortunate, rushing the hardening off process is probably something we've all done at some point - it really is important that transition to outdoors is done gradually.

It can take a couple of days before all the damage becomes evident, then you'll have to decide whether you want to start over with more seeds, or wait until the sunburned plants recover and put out new growth. If they are slightly damaged, chances are they will be set back slightly then recover. If the damage is significant, you may want to begin again - those whitened/burned leaves are not functioning ( providing any photosythesis to aid in growth), so if very burned, replacing may be best.

Gives them a chance. Never say never. I learned that the hard way. I harden my seedlings very early first by having an electric fan blowing softly on them. This is also important to dry up the soil and prevent fungus. Then I take them out in the sun for about one hour doubling that ever day for three days. Then I release them to the wild.

They may survive, but me being who i am, I would probably leave them and in the meantime start some new ones if you have time. Just in case. I hope they turn out ok for you and your kids.

ALL MY SEEDLINGS ARE DYING AND TRUNING YELLOW, i HAVE GROWN PLANTS ALL MY LIFE AND NEVER HAD A PROBLEM, THE LEAVES TURN YELLOW AND SLOWING THEY ALL DIE, AND MY MARIGOLDS JUST DRY UP LIKE THEY HAVE BEEN SPRAYED, EVEN MY GERANIUMS THE TIP OF THE LEAVES TURN YELLOE SOME WILL SPLIT AND WILLSEEM LIKE IT IS EATEN, i HAVE USED ALL NEW SOIL AND CONTAINERS AND HAVE USED THE SAME LIGHTING I HAVE FOR YEARS, PEOPLE USED TO ADMIRE MY PLANTS AND NOW I HARDLY HAVE A PLANT, I HAVE GONE TO THE AGRI OFFICE THE STORE WERE THEY SELL SEELINGS AND NO ONE HAS ANY IDEA, IT IS VERY DISCOURAGING WHEN I USED HAVE A WHOLE GREEN HOOUS FUL OF NICE PLANTS.

Voice immodulation disorder is a terrible thing. :)

Turning yellow sounds like a root issue. Is this a different type sowing medium/soil than you have used in the past?

I have some seedlings died especially the Cantaloupe and I don't know why? I thought is a fungus in the soil. I alsways experiment I prepared a solution equal to 3 teaspoon of Captan to a Gallon of water and drenched the pot soil not too much just to make the survace of the soil wet. I hope this will protect the remaining healthy plants.

Another option is to purchase seedlings from a nursery so you don't have to wait for seed to grow again thus getting behind in the season.

Veriria
This takes all the fun out of it. I don't make money from greenhouse, Vegetable Garden and tree orchard. It is a hubby although expensive one. Today I surprised my Girl with arrangement of roses and grand duke Jasmin. It smelled like heaven. All I produced. She appreciated it with a big hot kiss. I was proud of myself. That is what the hubby for.