Your code is failing because the bookmark doesn't enclose the entire row, though you don't need the bookmark as you can access the row directly through the content controls range.
Code:
Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
With ContentControl
If .Range.Information(wdWithInTable) Then
.Range.Rows(1).Range.Font.Hidden = Not .Checked
End If
End With
End Sub