The latest case was reported on Wednesday evening, the regional health authority in Hauts-de France said.
All eight children were admitted to hospital with severe digestive symptoms, such as bloody diarrhea, and five of them had developed HUS, the authority said.
Health authorities are conducting biological analysis in an attempt to identify the bacterial strain involved in each case.
They said there was no indication the children ate meals together and they have ruled out any issues with local tap water, which “can be used for drinking and for all everyday purposes”.
The infectious disease (HUS) is most often caused by E.coli food poisoning, authorities said. However, as the families involved had sourced their food from a variety of places, the origin of contamination is proving hard to find.
Food inspectors were investigating whether contaminated meat was behind the outbreak. Several butchers in Saint-Quentin were closed on Thursday, local news outlet L’Aisne nouvelle reported.
One butcher said all his meat, marinades and spices had been taken away to be checked.
Parents have been told to be vigilant and ensure strict hygiene at home, with authorities advising regular hand-washing, washing of fruit and vegetables, thoroughly cooking meat and separating raw and cooked food.