Mostly likely the caterpillars of the cabbage white butterfly. They can strip any member of the cabbage/kale family down to the veins over night. The only real prevention is to cover them with a fine mesh so the eggs can't get laid on there to start with.
Can confirm. Lost three brussel sprout plants to these little fuckers. I have broccoli that I need to plant outside but I’m not doing it until my mesh nets come in. Lesson learned.