I would already have paid for the 28-day passes There is
no 28-day Japan Rail Pass.
- Japan Guide: Drastic price increase announced for the Japan Rail Pass
https://www.japan-guide.com/news/japan-rail-pass-cost-increase.htmlSeeking advice in terms of whether to purchase passes for my kids: I'll be travelling with my two children, 2 & 4 years old. I suggest you do not purchase Japan Rail Passes for the kids.
According to your main plan,
you travel for specific JR sections (#)
where all the trains require seat reservation.
Even when your kid solely uses one reserved seat,
the JR regular cost for the kid to travel for those specific sections
should be about 23000 yen,
much less lower than the price
(before the October price rise) of child-rate 21-day JRP.
For the other JR sections,
you (almost) always can choose non-reserved seats,
in which case your 2 and 4 year-old kids can travel free of fees,
as long as you accompany them.
Travel period is 2-30 Dec 2023 From the evening of December the 28th,
Shinkansen trains may get crowded with New Year travelers;
I recommend you get close to the airport by December the 27th.
it's just very useful to have a pass where you can simply hop on and off without thinking too hard about payment, destination etc... I don't want the additional hassle of negotiating the ticket machines or queues at ticket offices and fussing over payments... You cannot "simply hop on" a train requiring seat reservation,
just with your Japan Rail Pass;
you have to get your JR reserved-seat ticket(s),
regardless of whether you use a pass or not.
All my hotels are near the main stations... How long do you stay at these places?
(E.g. Hiroshima for 2 nights)
Depending on your timeline,
JR area passes and regular tickets may be better cost-wise.
Hakodate to Fukuoka straight that will be a 12-hour train ride including stops at Tokyo & Okayama... I second comments by other posters
which suggest you never travel so long by railway with such little kids.
JapanCustomTours:
Consider flying. You travel during December,
so it may happen,
that snowfall makes flights from Hokkaido be canceled
while Hokkaido and Tohoku Shinkansen trains are still available.
Even Shinkansen trains may be delayed;
Tokaido and San'yo Shinkansen trains are not designed well snow-proof,
compared to Hokkaido, Tohoku and Hokuriku Shinkansen trains.
It sounds better to fly out from Tokyo than from Fukuoka.
Have you thought about so-called "open-jaw" flight tickets?
Anyway, listen to friendly remarks,
unless you are ready to waste your time and money.
(#)
Specific JR sections where seat reservation is necessary
: After Yamagata: Sendai -- Shin-Aomori
: Shin-Aomori -- Shin-Hakodate-Hokuto
: Shin-Hakodate-Hokuto -- Tokyo.