Hello JBake2024
It makes literally zero sense to purchase this very expensive tickets in advance. What will you do if the weather is bad and stormy, without any view up there? Would you still want to go?
We always suggest purchasing the Jungfrautickets on the day of travel or the evening before, once you are sure the weather will be fine the next day.
If you still want to purchase now, then purchase from the Jungfrau website. There, you can specify the tickets you already hold:
https://www.jungfrau.ch/en-gb/jungfraujoch-top-of-europe/buy-jungfraujoch-ticket/
When you purchase tickets from SBB, there is no need to mention the children or to purchase tickets for them. The Swiss Family Card is their ticket and valid when they travel together with a parent and the parent has a valid ticket.
Kind regards,
Roland