The anthropology of family relationships may, as demonstrated by Florence Weber, throw a new light on the current development of family structures. Taking this as our standpoint, we will examine the various logical approaches that are at work when it comes to the economic exchanges governing child upkeep that bind parents and step-parents together in the world of the extended family. These economic exchanges appear to be governed both by the obligations stemming from filiation and (or) co-residence – since the resources of the step-parents are included in the “bills” for child upkeep – and by an accounting reciprocity, which is both indirect and obligatory, due to the existence of a child who belongs to two “families” at once. Using a diachronic approach, we are able, through 14 case studies, to reconstruct the background to the modi operandi of the child’s upkeep, and to show that financial strategies, like the relationships to which they are applied, change as the child grows up and becomes the independent manager of his or her own financial requirements.