Hello mlar_go
The junior travelcard can be used when the parents have an Interrail ticket? YES.
And with point-to-point sbb tickets for the adults? YES.
The Junior Card (not the Swiss Family Card, it is not valid with Interail / Eurail, though) is only available at the SBB ticket office, not online. Certainly, you can purchase the adult tickets now and the Junior Card later. You must be in the possession of all tickets at the time you get to board one of the Swiss transit services.
However, for a 2-week trip in Switzerland, Interrail is not a suitable ticket as it is not everywhere accepted. You would rather look at the 15-day Swiss Travel Pass, or the Swiss Half Fare Card (plus point-to-point tickets). Both come with the complimentary Swiss Family Card; with it, children travel for free.
Kind regards,
Roland