I have view model which has another child model to render the partial view (below).
public class ExamResultsFormViewModel
{
public PreliminaryInformationView
Please make the below changes to your partial page. so it will come with your Parent model
//Parent Page
@{Html.RenderPartial("_PreliminaryInformation", Model.PreliminaryInformation);}
//Partial Page
@model Web.Models.Preliminary.PreliminaryInformationViewModel
@using (Html.BeginCollectionItem("PreliminaryInformation", item.RowId, true))
{
@Html.TextBoxFor(x => x.DateOfService })
}