The cabin is mentioned in the article, I had to use Google translate.
It's a log cabin built by the aunts and uncles which the children love to play in (my words). According to sources, when Emile woke up from his nap at about 4.30pm he wanted to go to the cabin, but the grandfather said he couldn't because they all (presumably his older children) needed to stay at the house to help him put up a fence. The gate was open and they didn't have eyes on Emile, who was playing in the courtyard of the house. The article said the cabin was uphill from the house, going in the direction of Col du Labouret. Col du Labouret is about 1.5 miles from the house according to Google maps, but the article doesn't say how far away the cabin is.