A brilliant alternative to the official Bullitt kid carrying accessories. The Holde box is a bit wider and comfortably seats two older children. At 26" wide it fits through most doors, but this is SF and we have "interesting" spaces, so do your own measurements.
The side panels are held on with a ratchet strap. When you need to carry wider cargo, they come off in moments and now you have a flatbed. You will be surprised how often this is useful.