We were required to have a passport on our Alaska cruise in 2012 (different line). I believe it is an "across the board" requirement for cruises that stop in Canada.
To get them now or wait....adult passports are good for 10 years. I'd get those. Kid passports are only good for 5 years--are any of the kids at an age where before your Alaska plans they would convert to the 10 year variety? If so, I might wait.
The ONLY reason you'd need a passport would be if something happened that prevented you from returning to the US on the ship--like a medical issue that required you to remain somewhere for treatment. The odds of that are not great, but I've seen it happen even in young, healthy people.
I don't know. If you are asking "should I get passports or should I spend the money on a cabana?" I'd say it was a no brainer--get the passports. On the other hand, if you are saying "This is a real financial hardship for us, do we really need to do it?" I'd probably say no. There ARE regulations in place that allow a US consulate or Embassy to issue an emergency passport for someone who needs to return to the US in just such a situation. It is just another hoop to jump thru at an already difficult time in your life. The cost of the emergency passport is the same as the regular passport and MUST be exchanged for a regular passport after return to the US, so there is no cost advantage or loss to the emergency system.