Thin them now, at least 4" apart, be brutal ;-)
But the more important question is, what is under that weighed down piece of paper in the middle of the row ???

I heard it would attract worms....
Brutal is the right word. It will be very difficult. Can I just snip them at the soil? or to I have to pull them out?

Snipping is better, that way there is no added stress to the nearby plants...

Worm attractant ;-)

I feel your pain. I have a really hard time thinning kohlrabi. I like eating the leaves of kohlrabi like you would beet greens or kale. You may want to try that with the ones you thin out.

That paper (or a piece of cardboard) will do something else nice for you. It gives a hiding place for the beneficial beetles that eat cabbage root worms.

Bellatrix

what the heck is a cabbage root worm!!! (scared face)

It's a pest of broccoli, cabbage, etc. I wouldn't worry about them too much, especially if you are growing organically. But better to give the insects that eat them some help just in case!

Bellatrix.

I could never bring myself to thin kohlrabi, carrots, beets properly - I always seemed to have clusters of plants interspaced between barren areas, and of course the two nicest plants in a section sprouted right next to each other. This year, at least for kohlrabi, I said forget it, I started them indoors like cabbage and broccoli, and planted them out at the proper spacing. I had such beautiful bulbs this year, I could have cried (I did burp a few times after eating them). My poor beets, on the other hand...

A few years ago, I had a limited number of Kolibri hybrid kohlrabi seeds, and didn't want to waste any of them by thinning and throwing them away. So I transplanted the little ones into a second row - and they did just fine! I kept them well watered for a few days; I did the same thing this past growing season, same good results. I probably wouldn't bother with the older, less expensive varieties, but Kolibri goes for a premium price, and I am frugal when it comes to my seeds.