You already have pass numbers: they are in your confirmation email. No need to activate the pass to book reservations, never do that.
I suggest using the VY app or website for the most reliable info.
2 year old: she would need a reservation to get a bed. Otherwise they don't care.
Side note: there are multiple issues with that line at the moment. 3 derailments, lack of maintenance, congestion due to the many many ore trains, etc.
Most trains are replaced by buses from Boden or Kiruna or you have to take another night train and change in Boden (7pm arrival in Narvik). Basically don't expect to get a direct train and be flexible. 1-2h delays are frequent.
My friend had to change plsns and spend a night extra in Narvik because of that (travelling in early September).