Thangpawl told the parents to take the children to hospital, and she called the council to report the incident. They put her in touch with animal control.
She said the black and white dog was "quite big and tall" and "really, really scary".
She was saddened that such an incident had happened in what was already a rough time for the group.
"We were gathering to support one another when we were in hard times.
"We thought we were in a safe place ... the kids, now they get frightened, which is not very good to experience."
The incident could have been much worse if the adults weren't present, she said.
The children were taken to Kenepuru Hospital and have since been discharged.
A dog matching a description given by the witnesses has been caught, said Porirua City Council's monitoring, compliance and customer manager Leonie McPhail.
"The council received information about the incident from a witness but have had no report from the victim or victim's parents," she said.
"We have impounded a dog [that was found roaming] that matches the description given by the witness but have not obtained confirmation if it is in fact the dog that bit the child. The dog isn't chipped.
"We will be following due process on the information we have now and anything further we might receive about the incident."