I have no specific answer, but at least I have flown with an infant in arms on CM (10+ years ago). I can’t recall any particular compassion about waiving fees for car seats and strollers. That rule seems to be written specifically against your case. Based on my experience with CM and their penchant for strict enforcement of all rules without exception, and no interpretation of rules to side with the customer (nor any empowerment for any agent to do so), I’d bet you’re going to have to pay. Maybe SJO agents have a different attitude and might try to accommodate a group of 3 pax in clase ejecutiva.