Create and edit child and grandchildern from within a parent form help please

I am using legacy forms to try and accomplish this. 

I have these tables in use:  Expense, Dates of Service, Invoices and Links to invoices. 

Expenses is the parent to Dates of service, Dates of service is the parent to links , and  Invoices in the parent to Links.  

1 Expense has many Dates of Service

1 Date of Service has many related Invoices

1 Invoice can have many linked DOS's (which need to connect to the related expense)

What I want to do is streamline my process and try to add the date of service AND then link that DOS to an invoice, all while staying on the Expense's table and at one time.  Currently, after creating the expense and date of service,  I then have to go to the DOS table in order to create a link between the dos and invoice. 

My process goes like this:

I receive and enter a new expense in the expense table.  Since each new expense has many different Dates of Service (DOS's) I have the DOS table. On my expense form I have an embedded grid edit report from the DOS table so I can add each DOS and its specific cost while I am entering the new expense.  So basically each expense has many line items, which are the dates of service. And then each date of service has many related invoices  which I connect via my links table. 

I log the expense details with the total due for all the dates of service listed on the expense, ie total expense balance due = $1000.00

The $1000.00 due is for 2 different dates of service, which I log via  the embedded grid edit dos report on the expenses form. 

Then.....

DOS #1  - $500.00 amount Due - DOS#1 has 5 different related invoice (each related invoice belongs to 1 patient account and then the $500.00 cost for the dos is divided and out among the related invoices and tallied in a another place to be displayed on each patients record.

DOS #2 - $500.00 (same scenario)

This allows us to account for the expense's for each date of service for the related invoice we have sent out.  

So how can I accomplish this all with the expenses table at one time? I was thinking that I could use a save and keep working button after the expense and dos details are entered?  I do have a relationship between the expense and links to dos tables but I would think I need to i want to link the dos to the related invoice using that relationship?

Hopefully I am just overthinking this and it is an easy solution. Any help is awesome.