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.
Prosecutor Eric Maurel said the two were also under investigation for “running an unauthorised campsite”.
On Thursday, 119 children were evacuated from the site, about 60 kilometres (37 miles) north of Avignon in the Gard region, after the Ardeche river burst its banks, swamping the campground in a fast-flowing, muddy torrent.
A German man in his 70s, who was with the group, was swept away by the floodwaters in his caravan or travel trailer, which was later found “empty and in pieces” near the river.
A fellow camper told France's BFMTV channel that as the river surged through the site, he could hear children's cries in the trees.
Maurel said local authorities had warned the German group earlier this week that the campsite was at risk of flooding after the heatwave that had left France parched ended with dramatic storms.
The youngsters were evacuated by the fire service and taken to a community hall where they spent the night, along with other holidaymakers.
Ten people were treated in hospital for minor injuries.
Rescue workers continued Friday to search for the missing man, who has not yet been identified.
According to the interior ministry, a total of 1,600 people, most of them campers, were evacuated as a precaution in the Gard and the nearby Ardeche and Drome regions.