Add a temporary question to the parent layer (outside of the repeat question) and republish. This allows both fields to be added. Then, go back and remove the temporary question and republish again. Only the repeat question is added (but the field for the temporary question remains in the hosted feature layer's attribute table).
In ArcGIS Online, open the related tables from the Data tab and add the new fields (with the same name and type), and then republish the survey.