The charity had received thousands of dollars from would-be foster parents in France to receive a child.
Spain is also seeking the release of seven of its nationals, who were crew members of the aeroplane chartered for the airlift.
In Chad's capital, N'Djamena, a prosecutor on Wednesday charged Jacques Wilmart, a Belgian pilot involved in the affair, with "complicity in abduction", before sending him to jail.
Wilmart, 75, made several flights ferrying the children between Adre on the Sudan-Chad border and Abeche.
The case has harmed Chadian-French relations before Paris begins its leadership of a European peacekeeping force in Chad.
|Sixteen Europeans are being held on suspicion|
of trying to remove children from Chad [AFP]
The peacekeepers are there to protect thousands of Darfur refugees and Chadians displaced by a continuing rebel insurgency against the Sudanese government.
Muammar Gaddafi, Libya's president, has offered to mediate in the crisis, but Nicolas Sarkozy, France's president, rejected the offer, saying he was in direct contact with Idris Deby, Chad's president.
Sarkozy, who has condemned the airlift operation by Zoe's Ark, suggested he would seek to have the members of the charity tried in France.
"I think that by clearly putting the Chadians and the French around the table, since the investigation was first opened in France ... well, you can imagine what my preference would be," he told reporters.
Mahamat Hissene, Deby's cabinet director, said the location of the trial had yet to be decided.
"Will they be tried in N'Djamena? Will the French authorities ask for them to be tried elsewhere? No one has raised the question yet, and we have no fixed position on the matter," he said in an interview with a French radio station.
Paris is under pressure from Chad's government after it emerged the French army provided the charity members with assistance in Chad.