It occurs to me that perhaps he never intended ANY of the children to leave the cellar, but by the time she had had the first two, who would have been about 3 & 4 when baby #3 was born, maybe she just couldn't handle it, or had a rough pregnancy/delivery, or maybe he decided the place was just too small for four people... and so he took the baby out of the cellar. Then the next two kids followed that pattern. Only the last one, the youngest, was left. It's possible that she wanted the youngest to stay... if you had to stay in there, it would be really hard to let your babies be whisked away one after another. Maybe she convinced him that she could raise the youngest one. Certainly by then the two oldest were old enough to help out, and the grandma might not have been able to handle a newborn at her age...?
Just pondering why some got let out and others didn't. I don't think it was a pick and choose so much since the ones he let out were all soon after birth - not much time to pick favorites, you know?