The six aid workers were detained in the eastern town of Abeche on October 25 as they were about to fly the children to France on a leased Spanish plane.
The defendants and their supporters have previously insisted that the aim of the operation was to rescue orphans from Darfur.
But Chadian and UN officials say most of the children were not orphans, were generally in good health and came from settlements straddling the Chad-Sudan border.
The French citizens face abduction and fraud charges in the Chadian capital, N'Djamena.
If found guilty, they could face sentences of up to 20 years of hard labour.
Three Chadians and a Sudanese national, accused of complicity, are also standing trial.
Anti-riot police, with helmets and shields, stood guard outside the packed court on the first day of trial on Friday.
'Rescuing Darfuri orphans'
Jeannine Lelouch, mother of Emilie Lelouch, one of the charity workers, told Al Jazeera that her daughter was attempting to take orphans from Darfur and help them.
"She [Emilie Lelouch] couldn't speak Arabic so she didn't know exactly what the translators could tell the families and to her. She was asking for Darfuri children and orphans.
"If these children are not orphans and not from Darfur it is not her fault. She only wanted children from Darfur to save them from the war.
"The Chad people must understand it was not against the people or children of Chad," she said.
Protest fast
The French nationals had been on a protest hunger strike for the past two weeks, saying they were being treated unfairly.
Nicolas Sarkozy, the French president, travelled to N'Djamena last month to secure the release of seven other foreigners and held talks with Idriss Deby, the Chadian president, over the case.
"We are a sovereign state, despite our poverty, and the French people should respect us," Ahmat Yacoub, supreme court spokesman, said on Friday.
"For our pride, our government's pride, they should serve their sentences here if convicted."
"There is politics disguised as law in this case," he added.
The case has embarrassed France, which supports Deby's rule in the landlocked, oil-producing Chad.
It has troops stationed in its former colony and is providing the bulk of a European Union peacekeeping force to be deployed in east Chad in January.
The trial continues on Saturday.