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In what concerns your question:
There is a full fare Saver Day Pass for Bern to Grindelwald, starting at CHF 59, plus a ticket for Grindelwald to First. Is the cost for the Saver Day Pass higher than CHF 78, then you are better off buying a point-to-point tickets Bern to Grindelwald and Grindelwald to First. Both can be bought at the SBB Travel Center in Bern.
Saver Day Pass – book earlier, travel cheaper | SBB
If you plan to travel in Switzerland more often, you should check the Swiss Half Fare Card. This card lets you get tickets at a reduced price. And it comes with the Swiss Family Card, which allows children 6–15.99 years of age to travel for free when accompanied by their parents.
Swiss Family Card | Schweiz Tourismus (myswitzerland.com)
Alternatively, the Berner Oberland Pass could be an option, too; all depending on how much you plan to travel in the area: Berner Oberland Pass
My general recommendation is to first plan your entire Switzerland trip and then, as the final step, procure the tickets. A Swiss Travel Pass or the Swiss Half Card can prove to be highly advantageous in comparison to single full fare tickets. We often see people here who have purchased a Saver Day Pass and want to exchange it for a Swiss Travel Pass because they came to the conclusion that, by the number of trips they have planned, a pass is more beneficial.