I agree - it looks like sunburn to me. Easy way to "harden off" (or, more correctly, avoid the hardening off process) your young plants is to cover them with floating row cover (sold as Reemay, Agribon, GrassFast, and other names; it's a nonwoven, lightweight covering that allows light and water through). Be sure to give it some slack when you lay it over your plants, and pin down the edges with garden staples. Provides some protection from both the sun and wind and gets things off to a good start. As FarmerDill said, your plants will likely kick out new leaves soon that won't be so tender, and everything will be fine.
Welcome to NGA, @hwrights
-- I hope you have a great gardening season!