In the Dardennes’ new film “Young Mothers,” parenthood is the focus. Though the two stick to their observational, naturalistic style for this one, they deviate from their usual single-story structure, instead following four teen mothers living in a state-run maternity shelter. The building is plain and administrative, but it’s clean and organized and safe, and you occasionally get the sense that for some of these girls, it’s the most secure home they’ve had in a long time.