You should just be able to cancel the two infants as a worst case. I'm not sure but I think I have avoided fees for infant changes (of date) in the past. No need to cancel the adult tickets and risk availability.

Alternatively could they travel under their current names (if they have passports). This would seem to have a parallel to a honeymooner who can travel in her maiden name ... from the UK at least - not sure about the US?