Are you sure about what you bought?
With a Swiss Travel Pass you do not need tickets for the Luzern - Grindelwald part, only for Grindelwald - Jungfraujoch. In your place I would just travel to Grindelwald, and buy the ticket there, once certain that the weather up top is good. Jumping the gun on train tickets is rarely a good idea in Switzerland.
Normally when you buy tickets in the SBB app for multiple persons, you get multiple QR codes. You should have entered “GA” as reduction, and “Junior Card” for any children, and received four QR codes.