This is an oldie, but a good question. Many would-be electric cyclist get a lot of misinformation from corporate marketing, insurance companies, governments, etc.. Because ebike technology is in its relative infancy (compared to say gas engine vehicles like a motorcycle) a lot has been learned in the last 5 years!
I ride a very similar to setup to what the original poster mentioned (Big Dummy Electric Bicycle) and have been riding it for over a year now to/from work as well as a half a dozen longer trips of 100km+ with total weight up towards that 400lbs mark you mentioned.
Now Gary has given you some good information regarding total power output but not more specifics regarding voltage/amperage/windings/etc. The total power output will very likely be regulated by local laws to just slightly lower than what is reasonable for your application. That's just how it works today in most of the world for uninsured vehicles.
So your choice in motor is more about how you use the maximum power output rather than selecting the power output. When a motor is manufactured a specific number of coils/turns of wire are wrapped around each pole in the motor of the appropriate thickness. By varying the number of turns and thickness of wire the same motor design can result in higher speeds or lower speeds. Most manufacturers will produce several common variations for targeting 20", 26" and 29" wheels.
When I setup my ebike, I bought the "high speed" motor winding. This is capable of propelling the bike up to 50km/h with a 26" tire. Please note this will exceed the legal limit everywhere I know of in North America, so off road use only. I have a limiter built into my 'Cycle Analyst' ebike computer which can then tone it down to the the legal limit for daily commutes (In Canada that's 32km/h 500W). The high speed motor scores high on the 'cool points' when you show it off to your friends, but you'll soon change your mind and wish you got the lower speed winding for daily efficiency.
The lower speed windings generally trade off speed for torque and/or thermal capacity (thicker wires - less likely to burn out on long uphill pulls). Also travelling at lower speeds will conserve battery power and allow you to make longer trips on the same battery. Wind resistance is not a linear force on your bike. For example on my bike maintaining 32km/h will take twice the power of maintaining 24 km/h, likewise maintaining 48-50km/h will take four times the power output (also exceeding most legal limitations). When hauling kids for a 4 minute 'drive', a steady pace of 20-24km/h is probably great.
So the related question here to motor selection is battery selection. Again you'll have a total capacity expressed as AH (amp-hours) which is the number of hours the battery would sustain a 1 amp draw. But equally important here is the voltage for the overall system. The most popular voltages are 24, 36 and 48 volts. 24 volts are usually under powered kits for heavily restricted European countries but is not really suitable for your application. 36v and 48v will be your primary choices. The voltage will be directly proportional to your maximum capable speed. Again I went with 48v when I built my ebike because its very 'cool' to show off the speed/torque. However, if I went with 36v system with a motor designed to max out at 32km/h at 36v I would have been more happy in the longer run from a practical viewpoint. That is because given the same size/weight of battery, a 36v battery will likely contain at least 25% more capacity (AH) as the 48v. This equates to longer run times or more distance between charges.
Again for practical use commuting with kids/trailers, you will be more interested in running a consistent reliable slightly slower electric bicycle than a 'hot rod' which burns through battery juice. So I would recommend a 36v motor/battery combination designed to just meet the maximum legal power output limits (Watts) for your local laws.
For the electric-assist nay-sayers out there, I believe bicycles will make a big come back in north america during the next few decades as practical devices (not just sport). Bicycles are efficient, small and user maintainable in many cases. Electric assist enables us to do more with a bike within a reasonable amount of time. Not everyone can afford to triple their commute times each day. I do recommend to everyone to improve their health, but most people need to move 400lbs around daily today and in a reasonable amount of time similar to driving. Electric assist does that. Plus electric assist bicycling is a "gateway drug" to other forms of bicycling! You'll find many people who own an electric assist bicycle also own other bikes for other less practical uses.