Any ideas?

I get them on my kale plants. I found that spraying the leaves - especially the undersides - every few days with water from the hose was enough to knock off or kill the eggs and caterpillars. It's a pain to do but it helped tremendously. I gave up growing other crops that are affected by cabbage worms but do like kale for soup so I keep spaying the leaves with water.

One year I used flexible PVC-type of plastic and insect barrier to make a dome over my bed of kale and kohlrabi. It helped until I took it off because I was concerned it was getting too hot inside the dome. I like the water spray better.

I'll follow along for other ideas.

Three steps used at my friend's organic farm:
1) Clean cultivation and rotation: You don't want to leave any residues for overwintering. Pick a spot where they weren't last season and weed or till before you sow or set.
2) Row cover. This really works well for us as long as they don't get in. Some type of fine netting should work as well, for hotter climates. The tulle or netting that brownrexx uses is also nice because you can see what's going on without removing the covers.
3) When the above fails, spray with BTK to kill the caterpillars and save your crop.
BTK is specific to caterpillars and won't harm non target beneficials (except maybe non target butterflies if they were reproducing on your crop).

I was going to mention the tulle netting because it doesn't have such a fine mesh
The cabbage worm butterfly is about the size of a nickel.
And the moth a little bigger.

We used to use 10mm netting, until I watched a cabbage white butterfly fold it's wings and squeeze through. Now I either use a 1.35mm mesh or 5mm netting which works fine, though if the leaves of the plants touch the netting the butterflies will happily lay their eggs from the outside.

The only problem with using fine netting is that it stops certain predatory insects like hoverflies and ladybirds from getting to the plants, so when aphids arrive they will reproduce with wild abandon and you can end up with a very bad infestation. I don't know if you have mealy cabbage aphid over there, but in hot summers they come in huge numbers and can badly stunt the growth of brassicas. Luckily there is a parasitic wasp which lays eggs in the aphids and is small enough to get through the 5mm netting, but in a bad year you really miss the voracious appetite of hoverfly and ladybird larvae.

10 mm is almost 3/8 of an inch.
Tulle netting is about 1/16th inch or 1 mm.
Just to give a little perspective.

Don't grow brassicas or floating row covers.

Spray with Bt. DE works well on kale and leafy things but it's hard to wash off of things like cauliflower. It gets trapped in the cracks. Neem oil helps too.

This is my bok choy from last year. I use either netting or a floating row cover, depending on what I have. I don't have an aphid issue so my netting size is no problem. I like the netting because I can see the plants. I get really nice brassicas with no sprays at all. I don't know what made the small holes in the bok choy leaves, maybe flea beetles, but it didn't hurt anything and I prefer a few small holes to pesticides.

Bok choy 2022 by Brownrexx, on Flickr

Bok Choy & Cabbage by Brownrexx, on Flickr

DSC00392 by Brownrexx, on Flickr

20190607_192047 by Brownrexx, on Flickr

20190607_191610 by Brownrexx, on Flickr