Half Fare Pass + Berner Oberland Pass?

Some things on your list get a better discount with the HFC than the BO pass, but it looks like you will not overspend like crazy, maybe break even or save slightly. There is not much in Gimmelwald, so during your time there you will probably go up and down a lot, it is not that expensive, but it will add up a bit as well.

I like the flexibility of getting on and off whenever.

Exactly, this is an added bonus. Just pay attention to areas that are not included in the BO pass and be sure to purchase tickets before traveling those routes!

@SwissNomad, confirming - the BOP will cover Gimmelwald right? It's not explicitly named on the BOP validity map for 2025. But it looks like that route from Stechelberg to Murren has a v structure to it and I am guessing that is where Gimmelwald is.

I did quick rounded math for you, so these numbers are off by a little bit, but it gives you a pretty close estimate.

Full Fare = 950 chf
Half Fare Card only = 600 chf
Reduced 8 day BO Pass + HFC = 620 chf

The BO Pass doesn't fully cover Harder Kulm, Eiger Express, Kleine Scheidegg or Schynige Platte so you'll have to pay supplement fares for Interlaken Ost to Harder Kulm, Grindelwald Terminal > Eiger Express > Kleine Scheidegg > Wengen, and Wilderswil > Schynige Platte at HFC pricing + Bern > Basel > Colmar on your way out, which comes out to roughly 210 chf in supplement tickets.

With the pass being within 20 chf it's well worth it, as certainly you are going to do a few things you didn't fully calculate here + the convenience with little kids of hoping on/off transportation without worrying about tickets (like the cable car between Gimmelwald and Murren, which you'll ride every day)

A side note, I'm assuming you know that with the new cable car situation between Murren/Schilthorn that they have reduced the cable car time to/from Gimmelwald to only once every 30 minutes, so make sure you keep that timetable on your phone to avoid missing it, since it's not as frequent as it used to be.

And I'd knock Harder Kulm off your list of priorities. The lines going up and back down are atrocious and the views aren't as good as Schynige Platte, Mannlichen, Brienzer Rothorn or Schilthorn. It's a waste of your time.

Hope this helps a bit!

@wanderweg thank you! Huge help.

Do I understand the discounts correctly?
For example, if Eiger Express from Grindelwald to Eigerglestcher is 42 CHF, would it be 1/2 with BOP (21 CHF), and then an additional 1/2 off with HFP (=10.50 CHF)?

No, that's incorrect. You don't get to stack discounts like grocery store coupons :)
The Half Fare Card gives you 50% off, as does the BO pass if you don't already have the HFC, but you don't get 50% off twice.

Grindelwald Terminal to Eigergletscher via the Eiger Express is 19 chf. SBB defaults to Half Fare card pricing, so that's what you'll get with your scenario. It would be 38 chf with no BO Pass or HFC.

The benefit of the HFC + BO Pass is that the HFC gives you a reduced price on the BO Pass (108 chf discount off) + half fare for anything you do that is not included in the BO Pass at all (Geneva > Montreux, Bern > Basel) -- The HFC will probably only save you around 50 chf in your scenario. Not a lot, but it's something. For some people with older children, it's a much bigger benefit as they get the Family Card for their kids to travel free, but your kids are so young that they travel free with no card or verification needed.

the BOP will cover Gimmelwald right? It's not explicitly named on the BOP validity map for 2025. But it looks like that route from Stechelberg to Murren has a v structure to it and I am guessing that is where Gimmelwald is.

Yes, exactly!