I will answer part 1 of your question. Lets get that far before the part 2.
You want to show grandchildren on the grandparent record. make sure that the Grandchildren (Serials) knows the record ID of the Grand parent. Look up the field probably called [Related PO receiving] from IMN down to serials. call it [Record ID# of PO Receiving].
Then go to the PO receiving table and create a field of type report link. call it [Report Link of serials] In the field setup select [Record ID#] on the left and then on the right navigate to your app and then to the field [Record ID# of PO Receiving] on the serials table.
Put that Report Link field [Report Link of serials] on the PO Receiving form and select to display the records on the form.
