I was going to post this in your other thread you started when you thought these were cherry tomato seedlings, but I see you' ex started a new thread.
-- I'll go lock that one and leave a link to this thread so we can keep things straight.
Now, I found an old photo of a purple kohlrabi, which is not red cabbage but would look similar at this stage. None of the seedlings in your photos have completely unfurled seedleaves, but they should end up with typical mustar/cabbage family seedleaves like this one pictured on the left:
I think of cabbage as cool weather crop. They sprout readily at high temps like 30Â°C as you may have noticed, but they prefer to grow in cooler more comfortable temp. (Like 50's-60's Â°F) How much cooler can the day/night temperatures be where you are?