Selamat malam. Maaf, senior disini dari 65. Ada empat dewasa2 biasa!
26/12 is 2nd Xmas day and a VERY heavy and difficult day to get RES or even tickets. Same for 1/1= tahun baru. Or maybe 31/12?
e.can do without any RES-use regional trains, change Basel SBB
f.avoid the very expensive Lyria direkt from Swiss to Paris-board it only in FR-Mulhouse or Strasbourg and go there again with regional.
a.€* from LON via b-europe. YOU need to have passcover nr-can generate that if you have mobile pass.
d+f. call SNCF is best way. perhaps they now tell you cannot yet do (major timetable change/jadal waktu on 11/12)
See also main overview by seewulf on howtodo RES, with link sent by anna dari Sverige.
Perhaps, perhaps in this case it may even be better to NOT buy pass but book NOW advance fixed tickets. Lah.
Hi 
1. we 1 4 adults - 32 , 35 , 36 & 60 years old , 1 seniors citizen 64 years old , 1 child (6 years old) ] would like to purchase eurorail pass for our coming December Europe trip .
2. We would like to go for 7 days in one month , flexible , multi country and first class
3. How we do our seat reservations and charges ?
a. 22/12 from Amsterdam to London
b.26/12 from London to Brussels
c.29/12 from Brussels to Paris
d.1/12 from Paris to Colmar ( French )
e.4/1 from Colmar to Grindewald
f. 7/1 from Grindlewald to Paris
4. From my abovesaid trip itinerary, should I go for mobile app reservation? Or paper reservation?
Your prompt reply is much appreciated.
Thank you very much 
First of all i would try to adapt the route to reduce the cost of reservations
Like Amsterdam - Brussels - London - Paris - Colmar - Grindelwald - Paris.
Next thing is which trains you wanna reserve?
Colmar to Grindelwald can be done reservation free if you stick within France to Regionaltrains (Note Interlaken Ost - Grindelwald is not covered by Eurail but you get a discount at the local ticket office)
Paris to Colmar you can take the fast TGV or the slower regionaltrains with a change in Strasbourg.
Here is a general Overview of how to get reservations
I recommend to provide more details which trains you wanna reserve. And as already said above the days arround Christmas are very popular and the Eurail Quota on some trains can be already sold out.
There are no seats for 6 passengers from AMS to LON on 22/12, but currently plenty of availability from Brussels. There is an hourly IC train from AMS to BRU.
There are 4 trains from London to Brussels on 26/12 with availability, but you need to check on transport to St Pancras
https://www.b-europe.com/EN/Booking/Pass#Outbound
