In October, Chadian authorities arrested members of the aid group as they sought to send 103 children on a plane to France. The group's members insisted they were driven by compassion to help orphans in Sudan's conflict-wracked Darfur region, which borders Chad.
But investigations showed most of the children had at least one parent or close adult relative.
The six were sentenced in the central African country in December to eight years of hard labor, then transferred to France under a 1976 judicial accord between the two countries. They were jailed soon after their arrival.
Because France does not have forced labor, the court in Creteil southeast of Paris was asked to adapt their sentences. On Monday, the court converted the sentence into eight years in prison. The Creteil court did not retry the case.
The lawyers for the workers, from the aid group Zoe's Ark, said they would appeal the ruling.
The group's transfer to France had sparked protests in Chad, a former French colony, with many Chadians decrying what they saw as special treatment for Europeans.
The case was an embarrassment for France, coming as the country was pushing to send a European Union force to Chad to protect refugees fleeing violence in Darfur.
European Union nations gave their final blessing Monday to sending the 3,700-strong peacekeeping mission to Chad and the Central African Republic.
EU military organizers have struggled to find troops and military hardware like helicopters for the mission, with many member governments claiming they were too busy with military commitments elsewhere, notably Afghanistan and Kosovo.