Try calling YouFirst as it seems a very specific circumstance, complicated by the facts:
– Redemption Booking
– Ticket “bought” from AA not BA
– Voucher used (poss. non-refundable)
– Confusion over twins vs. single infant
– Change in cost for infants since booking
I don’t understand why there is a need for an economy booking LHR-NRT; that smells fishy to me.