Hello all !
Child table is the best way to add different items in the same form/doctype versus filling a new form for each item separately.
Without child table : IMAGE-1
With Child Table : IMAGE-2
Steps to create a child table : -
1.Doctype1 > Maka a doctype and create field values, but do not enter the table col values. (like only the Supplier field in IMAGE-2 above).
2.Doctype2> Create another doctype but include only the field values which you want in child table (like Article Name,Quantity and Rate values in IMAGE-2 above).
3.Now in Doctype2 check the option “Is Child Table” which is present below the Module Name. Also check the option “In List View” so that it appears in table heading row.
4.Now in Doctype1 add a new field item, Field type should be Table and in Options enter the name of Doctype2.
Now when you open Doctype1 you can see the table and also the Doctype2 would not be visible under your module on desk but you can acces it via settings/Core/Backend.
Just click on “Add new row” and add your items !
Happy adding 
Cheers !
Yogesh.