>If a have a parent file with 2 fields as key (example: Customer Code and Invoice No.), how do I link them to my child file containing the same 2 fields among other fields? Also, how can I use a grid in a form for the child file with this?

Common practice is to structure your tables a little differently. Each table should have a primary key, which is usually an autoincrementing integer or GUID column. Your table structures could look like this:

* Parent table (Customers)
Cust_PKey  (Primary Key)
Cust_Code
CustName
....

* Child Table (Invoices)
Inv_PKey  (Primary Key)
Cust_PKey  ("foreign key")
Inv_Number
...

* In many cases, with Invoices you will want multiple Invoice Line Items per Invoice,
* so you'd want a child table of Invoices:
* Grandchild table (InvoiceLineItems):
InvLI_PKey  (Primary Key)
Inv_PKey  (Foreign Key)
Product_ID
Quantity
....

Using primary and foreign keys like this lets you link Customers, Invoices (and potentially Invoice Line Items) in the manner you want.