This is a healthy, productive plant. A few holes, tears or lost leaves won't ruin a plant's vigor. The inside holes are probably slugs, snails or flea beetles. The edges are probably being munched on by rodents, slugs. Plants are built to be rugged, hardy. If the damage begins to increase, you'll have figured out what to expect and you'll be on top of things. The cool thing is you are observant.
Brassicas are beloved nurseries for flea beetles and root maggots, you should check around the base of the stalk down into the soil for signs of larvae, maggots. These look newly transplanted? Plenty of time to get row cloth, I hope, to keep insects away from plants. Actually, I just went out and got some row cloth to do the same thing. I remembered an old vegetable garden where I'd had huge, gorgeous cabbages/cauliflowers that were great one day and dying the next. What I learned is there's a fly (moth?) that lays its eggs in the dirt around the base of a cabbage or the Brassicas. The larva feed on the roots and it will take weeks to even notice there might be a problem.
I try not to run out of row cloth, great stuff.