Most of the children were found to have come from families in Chadian border villages who were persuaded to give up their offspring in exchange for promises of education.
The Zoe's Ark members were detained on October 25 as they were about to put the children on a French-bound flight from the eastern Chadian town of Abeche.
French law does not recognise hard labour and the court in the Paris suburb of Creteil converted their sentences into normal prison terms of eight years.
The hearing did not examine the merits of the case against them in Chad.
The court said that, under French law, the case amounted to "illegal confinement of minors under the age of 15".
Eric Breteau, Zoe's Ark founder; Emilie Lelouch, his partner; Alain Peligat, the logistics chief; Dominique Aubry, a volunteer firefighter; and Philippe van Winkelberg, a physician, were all present at the court.
Lelouch raised a clenched fist in protest during the hearing.
Defence lawyers said before the hearing opened earlier this month that the Chadian sentences were not valid because they were imposed in what they described as an undemocratic state after a trial they said was not fair.
They had argued that the French court had a duty to repair "a terrible injustice" and immediately announced their intention to appeal on Monday.
Breteau went on hunger strike after he and the others were repatriated to France and detained in a prison outside Paris pending a sentencing decision.
But the French court ruled there had been no "blatant denial of justice" during their Chadian trial.