You book the return ticket and the child no-show for the return flight to BOS. You don't cancel the return flight which will lead to no refund because the fare will be recalculated to a o/w fare.

With a little luck there will be a cancellation or schedule change for return segment to BOS that will allow you to cancel and get a refund of the return segment.

There is no "credit" like you are used to in the US.