3 days 4 nights itinerary with base as Grindelwald

Q.
1. With only three days it makes more sense to me to spend time exploring the Jungfrau region.

2.Harder Kulm is for any age. There is a new playground up there as well. You can read more about Beatus caves with kids here: https://www.beatushoehlen.swiss/en/family-fun

3.No. Not with such a short time frame.

4.No The HFC will give you a discount on the BO Pass, but they are not cumulative.

As far as your itinerary, what is most interesting to your family? I would put those ‘big stones’ in first and add the other things where you can. Not sure that the Liselotte trail is that interesting compared to some others ( just my opinion from looking at parts of it when there). Day 4 looks a bit sparse to me.

Hard to squeeze much more in, but based on much experience traveling there

Day 3 - instead of Lieselotte to Hohenstein, walk from Männlichen to Kleine Scheidegg then return to Grindelwald on the Eiger Express cable way. The walk is easy enough for children. From Kleine Scheidegg (a food lunch stop, BTW), take the Jungfrau train to Eigergletscher, have a look around, then ride the cableway to Grindelwald. Excellent ride. Most of this is covered by you BO Pass but in 2025 some routes have a slight additional charge and I think the Eiger Express is one of those.

Day 4 - if you are up for another easy, beautiful walk, take the lift up from Lauterbrunnen to Grütschalp and walk to Mürren. Nice stop for lunch or a snack and a cute village with amazing views, From there, take the new cableway to the valley floor and a bus over to Trummelbach Falls. You can walk from Trummelbach back to Lauterbrunnen and see a “farm fridge” (and increasingly rare item) but I would not trade that for walking Grütschalp to Mürren. You can also take the bus (bus is included in the BO Pass) back to Lauterbrunnen and saunter to the Staubbach Falls. Then take the train back to Grindelwald.

Your proposed day trip to Interlaken looks too full to me. Geissbach is at one end of Lake Brienz and St. Beatus is mid-lake on Lake Thun. It is not fast to get between those two places. I’d skip everything in the Interlaken wish list and stay in the immediate Grindelwald/Lauterbrunnen/Mürren/Wengen area where there is more than enough to enchant and entertain. If you have a bad rain day, a trip on one of the lakes (included in the BO Pass) is one rainy day idea.

Thanks SwissNomad for answering my queries. Some clarity has set in.
Thanks Laurel for those wonderul insights. Great tip with keeping Interlaken as a back-up if weather turns bad. Proposed inclusion to the day in Lauterbrunnen sounds exciting.

With the changes in the BO pass for this year you might want to look instead into the Jungfrau Pass + Half Fare Card. And with your list I’d strongly recommend taking a day off Zurich and adding it to Grindelwald. You have a lot on your wish list and not enough time for it, and Zurich is going to feel like a bit of a let down after the beauty of the mountains.