How long was the hardening off process? Did you have high winds or freezing conditions?

I hardened them off up to 8 hours outside. The temps did drop below freezing over night. It has been windy but they are covered by 2 layers of mesh

Was that "8 hours" all at once? If so then that combined with the high winds and shock from the drop below freezing may have been too much for them to handle. Kale is a fast grower and may perform best directly planted by seed outside. Personally I'd plant more seed outside now and compare results later to help decide the best route to take next year.

What does the 8 hours mean? As in you set them out once for 8 hours, or that you set them out for a week or so a few hours at a time gradually building to 8 hours at a time?

Gradually yes

Since it's been a couple weeks, how is you kale doing?

They have bounced back some but still stunted. The seeds I DS are almost as big as the starts now