Looks like bad nutrient burn...that appears to be an AeroGarden they're growing in? If it is definitely don't use the nutrients that came with it!
Use plain ph'd water in the reservoir for a few days...then start adding tiny doses of nutrients once it's recovered.
It looks like too much fertilizer. You should get a ppm (also called TDS) pen to check nutrient strength. A pH pen is almost essential for hydroponics. The good news is that the new growth might be able to handle the fertilizer.