The only seedling problem I have had with mustard transplants is 'damping off'... which is blackened shrivelled collar at the soil surface.
If transplant seedlings are not robust, I suspect it is worth starting over.
A location would help. Where are you? Are you only growing indoors or have they been exposed to the outdoors/sun for an extended period of time? My kale plants have always liked intermittent moisture, as in the top few inches wet/moist. I water them twice daily and they get over 6 hours of sun. I'm in Northwest GA, and it's mid April and we've already hit 90's.. I'd suggest a broadcast 'shower' head, as to soak the foilage thoroughly. How many days have they been in the pods? You may want to expose them to some fresh air and direct sun, watering them prior. How mature are your seedlings (ballpark estimate)?
I'm in Norway, and i kept the seedlings indoors, partly as an experiment. I have also sowed some kale outside, but it hasn't sprouted yet. I don't think water could be the problem because they are growing in deep cups and i water often. The strange thing is that it only affects that type of kale, and none of my other seedlings, but that may be because kale is a cold weather plant.
pictures are worth 1000 words.
I wanted to post a picture, but i couldn't find the charger for my camera. The tips of the leaves start shriveling and eventually the whole leaf falls off.
in my experience kale and other brassicas are much better when planted outside..except maybe the more tender cauliflower
I hope my seedlings outside will germinate soon so i can see if it was caused by the indoor environment or something else.
Thanks for the advise!