You're always very vocal about your personal preference. But its not accepted by the mainstream. Anyone looking around the internet on what qualifies as a good cargo solution is not going to find hub motors unless they are low end bikes. There's very good reason for that whether you want to hear it or not.So my joke of a cargo bike with geared hub motor has about 7000 miles on it powered. Ha Ha! With one chain worn out so far. One $221 hub motor worn out. ~4000 lb cargo toted uphill 30 miles to my summer camp.
Commercially sold bikes have to be able to carry the load from the Pacific beach to the top of the Sierra without stopping. Most of the ebike market is CA, OR, WA. Geared hub motors will burn in that service, big warrenty problem. Thus the multiple mid-drive cargo bikes sold. Geared hub motors might be perfectly suited to Maine. OP hasn't burned his/her motor yet on his/her Radwagon. No 1000' rises in 30 minutes? Just OP is concerned about stability of the Radwagon, or maybe the stopping power of the brakes as is.
Agree with you totally about inadvisibility of carrying a 6 year old on back of a MTB or cruiser frame. It would be more stable with a seat mounted on the front, a la 2 wheel butcher's bicycle of Arizona (not the 3 wheel copenhagen bike with huge bin).
Those big box kid hauler's from NET & BEL yuba & R&M are pretty safe, but must be **** to ride into a headwind. Expensive too with the wheel in front of the box and the steering in the back.