on thhe visibility fornt, surely a bike where the wide part is right where you can see it (ie just in front of you) rahter than behind like on a trikidoo would be easier?
I have to go through wuite a few narrow gates on my route (not the same as a canal path, i grant you) - one where there is a ditch either side which has been worn away by cyclists not going throught the gate over the years, and I have always manged it ok (and spatial awareness is NOT my strong point )
the Kangaroo seat is only suitable for 6 months+ (so they say), but I am not sure how you'd be carrying a baby any smaller ona trikidoo? unless in a cycle baby seat, which you could fix onto the rear frame of the Kangaroo (am clueless about cycle seats, btw, so sorry if this is not practical)
I have to say, depending on the jolt-iness of your route (and assuming you wouldn't be zooming around at 90 miles an hour ), I'd be tempted to put a baby in a car seat wedged in behind the Kangaroo seats - I reckon you could get it fixed down quite well - there's loads of room in there. If I had another baby (and if I was tempted to get back on a bike while said fictional baby was still small) then that is what I would do, then move one of the older girls (prob dd2) onto a rear seat when the baby went into the Kangaroo baby seat.