Possibly the caterpillar of the cabbage white butterfly, though they tend to chomp away and eat the whole leaf. Could also be flea beetles as they make small holes. As you will be eating the cabbages you don't want to spray anything toxic on them. You could use environmentally friendly washing up liquid such as Ecover in a spray which will kill them off. Spraying with a garlic compound before there are any bugs deters them as, whilst you can't smell the garlic after a few minutes, the bugs can smell for around a week. It is a systemic so going into the plant - but your cabbage wont taste of garlic!