Create child/nested bookmarks using Excel vba

I have successfully created a new PDF and added bookmarks to it.  I am trying to create child or nested bookmarks, but can't seem to figure out if this can be done using vba.  The code I am using to create the bookmarks is;

PDFPageView.GoTo (ws.Cells(irow, 1).Value)
AcroApp.MenuItemExecute ("NewBookmark")
x = PDBookmark.GetByTitle(LiquidPDF, "Untitled")
x = PDBookmark.SetTitle(ws.Cells(irow, 2).Value)

This works well, but really need to nest some of my bookmarks.

Thanks,

Dave