When I had this happen, the underlying principle was that the person who the ticket was booked for was the person travelling.
A few bits of paper to confirm that this was indeed the case and they were happy to make the changes. Again, until they had the pieces of paper, they refused to say that they could do anything as name changes are not allowed.

Could you not keep the seats, remove the infants, refund their tickets and then add them again? This is very doable by an agent (as long as they know what the are doing!)