Fetch values from childtable to a doctype

I have a child table vehicle in customer doctype. While creating a Customer I add values into this child table. Now when I am making a sales invoice and prints it, I need the info of the childtable(vehicle) in the sales invoice doctype. I do not want another child table to link the info in invoice as I need each value to be printed in the print format one below the other. So do I create some custom field and type into the fetch from field? If yes, then what would be the code?