K2 Tools Calendar doesn't consider child categories of a specified category, and this is quite frustrating. Choosing "none" is not always an option because of number of reasons.
Is there any workaround besides hacking the K2 Tools module?
Or maybe any free extension with same functionality?

Thanks

Zura

P.S. I installed the extension "additional categories" but it doesnt work with K2 Tools calendar...

There is B2J Calendar but I do not know if it is free or not.
I would strongly recommend against hacking core files since it can lead to unexpected results.

B2J is paid.
Anyway, i traced necessary places in code and edited(sql selects at couple of places). And it works. Now when selecting parent-category(without any direct items) - child category items are processed by calendar as well. At least for 1st level child categories it works.

Nice to hear that you solved the issue.
Keep in mind that you need to keep a backup of these edits since they will be overriden when you update K2.