Look like cabbage aphids to me. A very common cabbage pest. Lots of Google Images of them you can compare to yours to know for sure.

As with most aphids they can be blasted off the plants with water, brushed off with a paint brush, squished by hand, dusted or sprayed with various pesticides - both organic and synthetic - Your choice. But you have to hit the bug itself for them to be effective as they aren't affected by any of the eat-to-kill pesticides. Well established plants can tolerate a fair amount of them with little damage.

They are attracted to high does of nitrogen so avoid excess N fertilizers.

Dave

What Dave said. Another thing though. Whether you use water, insecticidal soap, neem oil, or something else; you have to stay diligent. Instead of the recommended weekly treatments on the bottle for heavy infestations, try every 3-4 days for the 1st couple weeks until you can bring down the populations. Aphids reproduce at very rapid rates, so really attacking them is key, IMO. You could also release some ladybugs or lacewing larvae to clean up the guys. If you do, no pesticides.

Good luck

Kevin

Ladybugs are really a good control if you have them. They go to bed around Oct. 15 here, and all of a sudden pristine brassica plants get populated with these.