The six French nationals standing trial could receive lengthy prison sentences if convicted.
On Wednesday the authority that sends suspects for trial decided that the case should be heard in a criminal court, rather than a lesser court.
Abdou Lamian, a lawyer for the accused French citizens, said he would be lodging an appeal with the supreme court against the decision to send his clients to the criminal court.
'Infringing on rights'
Lamian has alleged that the Chadian authorities were rushing the trial with "unusual speed, infringing on the rights of the defence and legal procedure".
The six face criminal charges including attempted kidnapping and fraud after they were detained in late October for trying to fly the children, aged between one and ten, out of the African country.
They have denied the charges.
The accused French nationals are members of a humanitarian activist group called Zoe's Ark.
The other three - the mayor of the town of Tine, on the border between Chad and Sudan, his chief official and a Sudanese refugee - are being tried for complicity.
The defendants, who were taken to court on Thursday to be informed of the trial date, could receive sentences of between five and 20 years of hard labour if found guilty.
"The farce goes on - apparently it's the 21st [of December], so we'll be there." Eric Breteau, the head of Zoe's Ark, said as he emerged from the courthouse.
The accused began a hunger strike on Saturday to protest their innocence - although they are allowing themselves water and cigarettes.