A third child died on Monday morning after falling into a pond near Paris on Saturday.
The eight-year-old girl had been admitted to hospital in a critical condition. Two other children, aged three and eight, died on Sunday.
One other child was still hospitalized on Monday, after a total of five children aged between three and eight were discovered unconscious in the water on Saturday, having apparently been left unsupervised.
The pond is located close to a cheap hotel on an industrial estate near Saint-Ouen-l'Aumone on the northeastern outskirts of Paris.
The children were from three different families and were temporarily in care while their single mothers underwent psychological treatment.
The prosecutor in charge of the investigation Eric Maurel said on Saturday that nobody had seen the children enter the water.
Police were called by a hotel employee who was alerted by another child.
Mickael Flour, who lives with his family in a caravan close to the pond, said children often played by the water.
"They like to throw sticks and stones," he told AFP. "We would often tell them, don't go down there, it's dangerous."
Another local, Sylviana Ternus, 29, added: "The children were often on their own, with no adult."