The feeding damage (and the early nature of its appearance, on plants this young) suggest slug or snail issues. You can check on the crop at night, when they'd most likely be out feeding (so the sun doesn't dry them out during the day, or when predators are more likely to discover them). If present, this page gives a few ideas for control and prevention: https://extension.umd.edu/resource/slugs-and-snails-vegetables

This page goes into a bit more detail, though it's written more with decorative, rather than edible, plants in mind: https://extension.umd.edu/resource/slugs-and-snails-flowers

The floating row cover you've been using can certainly help shield crops from an array of pests and strong, hot sunlight. Unfortunately, it can also sometimes trap pests inside that are already present, or keep natural predators from finding them as easily.  In this instance, it may also be hampering some air circulation that will dry the surface soil out faster, keeping the area more humid and hospitable for slugs and snails. You could experiment with leaving the cover off on cloudy but dry days, though it risks allowing other pests (like cabbage white butterflies, whose caterpillars can eat kale) access to the crop. (In that example, though, it's unlikely the butterflies would lay eggs on a plant that tiny, since it can't support the full development of a caterpillar yet.)

For now, see if you can spot any slugs or snails and try trapping them or hand removing them before resorting to baits or other chemical intervention. Leaves don't heal damage, but if new growth appears unscathed and stays that way, the problem has likely resolved itself. No need to remove any damaged leaves, since they're still able to photosynthesize and feed the plant.

Miri