In one summer camp in Saint-Julien-de-Peyrolas, 119 children were evacuated.
The Gard government also said four German children were hospitalised for hypothermia in Bagnols-sur-Ceze.
Heavy floods have forced the evacuation of 1600 people - majority campers - in southern France, officials say.
The missing person is a 70-year-old German man who is feared to have been swept away by floodwaters while inside his caravan, police said.
Rescuers believe the man sought shelter in his camper when fast-flowing water burst the banks of the nearby L'Ardeche River, but the vehicle was swept away.
Divers were taking part in the search for the missing man, an AFP journalist at the scene said.
The mayor of Bagnols, Jean-Yves Chapelet, told BFM-TV many Dutch tourists were among 350 campers evacuated there.
A total of 136 German children on a school trip were trapped in the campsite and had to be rescued.
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.
Rescue operations were in place at three different campsites in the region on Thursday.
'No one has suitcases.
Several parts of central and southeast France are on flood alert after a blazing heatwave ended abruptly in storms on Thursday.
Numerous roads in the area remained cut off. Authorities warned that the flooding would take time to recede and urged people to be vigilant.
Around 17,000 homes in the area are without power.