I don't have any children or grandchildren. I have carried up to 80 lb on the back of my yuba bodaboda. Most of the weight has to be ahead of the rear axle, so a 80 lb rider would have to cram forwards. Yuba sold a padded shelf and a set of handgrips that bolted to the seat post. It came standard with footrests and spoke covers to prevent pinching of fingers. It has been discontinued.
Blix packa claimed 100 lb rear cargo in 2020 but it doesn't have foot rests. You can look at other stretch frame cargo bikes, xtracycle, pedego, kona, m2s, magnum, giant momentum, eunorau, yuba, to see if you can equip them with footrests, a padded shelf, grab handles, spoke guards. I'd be suspicious of that much weight on 20" tires like tern. My bodaboda bangs the rim sometimes with 80 lb cargo and 55 psi in 2.1" x 26" tires. Some stretch frame cargo bikes come with 2.4" medium pressure (55) tires.
Over 80 lb you really need an electric tandem. I don't know of anybody that makes one.
750 w is rather wimpy to get 330 lb or more up a steep hill. I did fine with 1300 w and 1000 w geared hub motors, but the new legislation has dried up the sources of those motors in the US. The mid drive crowd will give you a lot of lies, but only mid drives with a rear sprocket bigger than the front (46 usually) will climb a steep hill better than a geared hub. Mid-drives will climb longer, geared hubs overheat with an hour of slow dragging with max weight up a steep hill.
Really with the wimp power limits mandated by the 3 class legislation, you are better off now buying a gas scooter with a <5 hp motor. Except for your and the rider's ears. Wear earmuffs if you like music. I can hear the **** scooters blatting around from a half mile away.
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