Without any actual information and just for fun speculating, maybe the toddler was originally flying as a lap baby, then the family decided to get her her own seat to make it a better trip, but because she’s a toddler it’s easier to buy a ticket for an adult rather than a toddler on her own and dealing with all the cross referencing business. So that all happens and then this situation.

but who knows, there are lots of ways they could have gotten there, and either way there’s a clear set of failures. Even just those emails at the end, nevermind all the staff throwing their hands up.