The easiest way to get train tickets is to use the SBB app.
You can buy tickets for all four on one phone. Just set up the app for yourself first, and then add the other passengers under "co-passengers". There you can indicate for everyone that they have the half fare card. You do not need to add any HFC number or so, as this will be checked on the train when the conductor comes, not during purchase. (You can in fact buy half price tickets without even having the HFC yet...)
The 11 yo does not need a ticket, as you have the family card.
Anyway. If it is all to complicate just buy tickets at the station. That is pretty straightforward. This is mass transit, used by everyone here to go about their daily business. It is not as complicated as you may think.
On last thing: You can buy tickets between any two points in Switzerland, regardless how many trains, trams, busses or other means of transportation are involved. Don't make that mistake I saw a group of tourists make yesterday, where they first bought a ticket Wengen - Lauterbrunnen, and then queued up again to buy a ticket Lauterbrunnen - Murren.
They could have bought Wengen - Murren in Wengen while they were waiting for the train there without issue.