Official website says:
"STOCKHOLM, Swe. - December 16 2013 - We are very happy to present our latest project "Nurture". It's a small simplistic game where we invite you to return to the world of Shelter every day to take care of your young. In this intimate and slow paced experience you will feed your cubs and seing your small family begin its life.

Nurture is a free experience that is accessable to anyone that owns a copy of Shelter. Simply select "nurture" via "extras" in Shelters main menu."

Any informations about a possible release on GOG? Thanks!

edit: New patch 2.0.4.10 with Nurture is on GOG!

I haven't tried this yet but by that description it seems to be some kind of freeplay. Is that true? Basically survive as long as possible, possibly with as many children as possible or anything else (or none of them)?

Nirth: I tried it few times. You have your cubs and they need some food of course. It should be 30 days long. Each day you can get outside and explore. My first try failed, because I was away for a week and when got back into game, everyone passed away - time is running no matter if you start game or not.

At the beginning you have not much to explore, there is a fallen tree that blocks a path. But some days later that tree will disappear and you'll be able to get further. If you will do well, you'll see as your cubs prosper and grow up :)

Interesting. I'll try it after I complete the regular campaign.

Has any one else completed this mode yet?

I'm working on it. I'm on day 14 ... and suspect I have played too much every day. 3 pups have already left. (running away, but really not getting anywhere until I quit becuse the map is closed) and I distinctly remeber hearing it should take a month. so... yeah. huh?

*!
Day sixteen and the last cub left. are there more areas if you ... have them stay longer?