Pick them off and destroy them.

Insecticidal soap. It is pure soap with no additives. Spray in early morning or early evening so the leaves don't burn or stay wet at night

Insecticidal soaps and horticultural oils are contact insecticides so you MOST spray everyone of your new found friends. It gets pretty expensive.

Also spraying does not get everyone unless you individually spray each leaf. I noticed this problem on mine this evening but also noticed that few were on the top side of the leaves. I imagine that the few that were on the top of the leaves are bird or wasp food as I saw one wasp carrying one of the worms away.

There are three moth larva that usually go after any member of the cole family of plants the Imported Cabbageworm, the Cabbage Looper, and the larva of the Diammondback moth. How to control them depends on a number of factors such as the time you have, the number of plants you have, and the amount of damage you are willing to stand.
The simplest method of control is hand picking, but that is also the most time consuming and often the least effective since you do not see all the wee buggers always. Spraying with Insecticidal Soap can be a bit better if you can be sure you spray all of the wee buggers, but you do need to spray them since the IS has no residual action. Next up would be Bacillus thuringeinsis - Kurstaki a bacterium that the larva must ingest for it to work, but that is only effective while the larva are fairly young. Then, in order of overall toxicity, would be Neem Oil products and the pyrethrins which should be relegated to a last resort use since they are broad spectrum poisons.