Goblin snatchers came to my fortress yesterday. One got killed before entering, one stole a child and got away, and one bagged a child and got killed. Some time later, I decide to check on the remaining child (I only had two) and see how she's handling loss of brother. But when I tried zooming to it, the game kept zooming to my stonecrafter. I thought that was weird, but I remembered a tale about a peasant living in a bag, so I checked the stonecrafter's inventory. Sure enough, it contained the child.
I tried building the bag, but it was unavailable. I tried marking it for dumping, thinking that the stonecrafter would drop it and I could unmark it before some other dwarf dumps it. The stonecrafter just continued working normally, without dropping the bag or chasming it (and yes, I waited for him to finish his current craft). So, I searched the forums and found out that trade depot can save the child (I'm not sure how, maybe I could sell just the bag?). Unfortunately, the child died of thirst before merchants arrived.
So, is there any other way to release bagged children? The parents of this one handled loss of both their children pretty well, but I fear I may get some tantrums in the future if this happens again.