Select multiple child records from parent record

I hope someone can help me.

I have a parent table called "Invoices" and a child table called "Costs".

Costs are populated by a team when we receive credit card statements and are essentially the line items on the statements.

What I'd like to be able to do, is to create an invoice in the "Invoice" table, and once saved, pick costs from the "Costs" table to be aligned to the invoice.

I'd also like a way to only select costs that have not yet been aligned to a invoice (perhaps if a particular field is blank??).

Is this possible?