This is a $2500 CAD proposition, so the J-wife will nix it, and I'll be lucky to get a trailer; however, if I can convince her, a longtail bike is the better option to box-bikes or trailers. If I can get her to keep living without a car...
Why not the others?
- trailers may be cheapest, but are a bitch to haul, put your kids far out of communication, and they tend to find it boring
- box bikes are great, if you have a garage, never climb a hill, never need to lift its bulk, and are of precisely average height as they are one-size (i.e. you are in Amsterdam) unless they are bespoke and cost a fortune
- box bikes stick your children far into the intersection before you can see into it
Why a longtail?
- multi-purpose for hauling kids and/or cargo
- easy to switch out any of the components as need or destruction requires
- does need a better rider than the others, but I am
- much easier to store than the other options
There are very few as the Trek Transport and Kona Ute are discontinued, and neither was ideal in any case, except the Ute's price. I have settled on a Surly Big Dummy, but the deciding factor is my height of 6'1"+: the Dummy comes in more sizes. Worth noting that Xtracycle's also discontinued the Free-Radical, which had its own issues. Other options are the Xtracycle Edgerunner 27D (too small), the Madsen Cycle or Yuba Mundo (one-size and poor components), and Bike Friday's version (small platform, proprietary parts and Bike Friday's complicated ordering system and no chance to test ride).
The Surly Big Dummy would never get sold after you stop putting children on it, because it's so fucking cool. It's a long mountain bike frame, indestructible, so one could load it up for any purpose or duration. The only thing I would regret on the Dummy, besides the loss of a few thousand dollars, is not having Madsen's box, which wouldn't fit: it saves futzing with cargo straps and child seats. Might be a way to make one's own.