We initially thought that a trailer would be too bulky for Tokyo, or that it would be too heavy to pull along behind a bicycle. This combination of electric small wheeled bicycle (National Panasonic model) plus high quality trailer (a Burley "Solo") has proven to be extremely versatile. The electric motor is powerful enough to assist my wife up any hill (and there are some serious hills in our area). The trailer is made for a single child, but we use it to carry both our children (now five and three). It also can be removed from the bicycle and wheeled around like a pram if the child/children are asleep. There is a cover for the trailer, so the children stay dry in any weather, and also space behind the children for about two large bags of shopping, which is also protected from the elements. Most bicycle trailers can also be folded quite easily when not in use, and the Burley Solo is no exception. It can be folded and chained to a post near the school allowing the bike to be used by itself until pick up time. Our school commute is about 3km which takes around 15 minutes at a very leisurely pace - a slow jogging pace of 5min per km. The longest trip I have taken the electric bicycle with trailer was about 6km, and the battery worked fine all the way with some power still left at the end of the day.
Electric bicycles abound in Japan, and very good ones at that, including some with regenerative braking. But it can be difficult to purchase a bicycle trailer in Japan as they are yet to really take off. We bought the Burley Solo from REI, the US outdoor store online. WIKE is another maker of quality bicycle trailers based in Canada. They sell direct and will also deliver to Japan and other countries.
Recently bicycles seem to be experiencing a revival of popularity in Japan, as in other countries. Bicycle trailers also are attracting a lot of interest. One of the country's largest delivery companies, Kuroneko Yamato has begun to use large bicycle trailers behind electric bicycles also. In fact, it was seeing their set-up that made me think that an electric bicycle might enable my wife to pull the trailer also. Up until I bought the electric bicycle, the trailer was just too heavy for her to pull behind a bicycle.