About 750 people in total - including many British holidaymakers - were evacuated in Gard, mainly from campsites, district officials reported.
Rachel Buchanan, from Oxford, was left with a "frightening" drive through floodwaters that had hit a site near a river in the neighbouring Ardeche region.
The public prosecutor for the region confirmed to AFP that the president and vice-president of the association that brought the children from Leverkusen to the village of Saint-Julien-de-Peyrolas were being held on suspicion of causing unintentional injury and endangering the lives of others.
Collomb, the interior minister, said in a statement that 1,600 people were evacuated as a precaution in the Gard, the Ardeche and the Drome regions.
Police said the caravan was later found "empty and in pieces" after being swept away by water that overflowed from the Ardeche River.
"The level and speed of the river today was extraordinary - we woke up in about a foot of water", she told the BBC.More news: Everton sign Barcelona defender Yerry Mina, Andre Gomes and Bernard
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The Gard administration also said four German children were hospitalized on Thursday for hypothermia in Bagnols-sur-Ceze, a town on the Ceze River.
"No one has suitcases".
Supported by four helicopters, more than 400 firefighters and paramilitary officers were mobilized for the rescue operation.
Authorities warned that the flooding would take time to recede and urged people to be vigilant.
Numerous roads in the affected area remained cut off as night fell.
Pictures showed rescuers wading through flooded campsites with damage to tents, caravans and bicycles.