Hi Sara and thanks for your question and pictures of your brassica plants. The larger ones actually look pretty healthy although I see some slug damage and I finally found the ants in the pictures. I would first of all pull out one of the dead/dying plans and look for root damage, white ant eggs or other signs that the ants are actually affecting the plants. Keep an eye open for round pearly slug eggs as well.

If you are convinced the ants are affecting the roots and you see a whole lot of them, you might try some of the different ant baits from the store. Different ants like different baits, so you may have to try several to find the one they cluster around. Read and follow the directions carefully. I also tend to disturb their mounds when I find them in my vegetables, as they have to expend a lot of energy to rebuild and sometimes just move on.

Good luck and fingers crossed for your kohlrabi and brussels sprouts!