I'm currently riding an ancient mountain bike with a Topeak seat on the back but need to upgrade since my kid is almost 5 and 42lbs. I live in an apartment in Brooklyn and plan to commute him on the ferry each day, so the bike needs to be short and maneuverable enough to fit in my elevator and to get on and off the ferry, but big enough to haul one kid and a few more things.

Seems like options for shorter cargo bikes are few. I'm looking at a Yuba Boda Boda V2 (6'2" end to end) and Benno Carry-On (6'1") but I'm very interested in a conversion kit if there's a shorter one out there. I ran into a guy in the park today with an Xtracycle Leap who said he thought there was. Ability to add an assist at some point would be a plus but I'm looking to stay under $1500 for now. Any ideas?

If I get the Carry-On I'll need to get a deck, pads and mini-handlebars since they're not yet making them (coming soon in 2018 but I'll need them as soon as I get the bike).

Thanks!