A mid drive or light weight front hub, I have a tsdz2 on my Kona ute.
BBS01 is an option or even a nice lightweight hub motor like the bafang g370 which is only 1.7kg. I use both my ute and roadrat with g370 for towing up to 200lbs with my CF trailer.
There is also a proper extracylce on ebay at the mo as well also kitted out for kiddy carrying and cargo load, it is in london but not a cheap bike. One could also add a kit.
A cargo bike of some description is the way to go if you have room for one, they are simply designed better to carry the extra load and have the seat adaptions for them.
For stability a three wheeler may be better but the longtails or front load cargo bikes may be easier.