Two options I can suggest from my own experience

1) as others have said, try the travel insurance option.
(although I find all insurance a con and they always seem to weasel out of paying out)

2) explain the situation to BA on a phone call and move the dates to 2020 for that baby moon. They will probably allow duration changes along with new dates. Once +1 arrives simply call again and add them as a lap infant. Beats losing the deposit
small snag the hotel price may change as will the flights so maybe look at the low fare finder for the cheapest flights and go from there.

and of course congrats on the baby news ^