In Japan, the JR companies in their grandmotherly kindness permit children under 6 to travel for free on all trains... except:
Fares and Charges for Infants and Babies
Infants and babies travel for free, except that they need child's tickets in the following cases.
When three or more infants accompany one adult or a child (tickets are required for the third and any additional infants).
When an infant or baby occupies his or her own reserved seat, Green Car seat or berth, etc.
When an infant travels alone. [Ed.: How does that work!?]
So if I'm traveling with two infants on an all-reserved train, I need to buy a ticket if I want to ensure that they have seats. Fine, that makes sense, but here's the question:
If I want to try my luck on an all-reserved train that's likely not full, can children under 6 board without a reservation and sit in seats that happen to be free? Or am I required to hold them in my lap all the way, even if there are plenty of free seats around?