OrdersBindingSource.DataSource = From o In school.OrderSet.Include("tblDetails") Select o
    DetailsBindingSource.AllowNew = True

В набор AssociationSet "FK_tblDetails_tblOrder" добавляется связь
или связь удаляется из этого набора. При наличии ограничений на количество
 элементов необходимо также добавить или удалить соответствующий "Details".

<Association Name="FK_tblDetails_tblOrder">
          <End Type="SchoolDataModel.Order" Role="Order" Multiplicity="1" >
            <OnDelete Action="Cascade"></OnDelete>
          </End>
          <End Type="SchoolDataModel.Details" Role="Details" Multiplicity="*" /></Association>

Private Sub DetailsDataGridView_UserDeletingRow(ByVal sender As System.Object, ByVal e As System.Windows.Forms.DataGridViewRowCancelEventArgs) Handles DetailsDataGridView.UserDeletingRow
    school.DeleteObject(e.Row.DataBoundItem)
    e.Cancel = True
  End Sub

Dim o As New Order
    o.NUMBER =  1 
    o.ORDERTEXT = "Один"
    school.AddObject("OrderSet", o)

    Dim d As New Details
    d.DETAILSTEXT = "11"

    Dim d1 As New Details
    d1.DETAILSTEXT = "12"

    o.tblDetails.Add(d)
    o.tblDetails.Add(d1)
    school.SaveChanges()

    school.DeleteObject(o)
    school.SaveChanges()