The findings were announced by Renu Tungkhajiwangkul, deputy secretary-general of the PM’s Office.
Upatham Nisitsukcharoen, managing director of Right Man Co, which built the structures for the event, said too many visitors hit the arch while walking past, making it unstable and causing it to collapse.
He said his firm was sorry about the accident and would shoulder all medical expenses of the mother and her two young sons.
He said the boys had completely recovered from their injuries while the mother was still recovering but could now walk.
He added that the company had insurance to cover accidents and would seek additional compensation from the insurance firm to give to the family.