A mid-drive is better on paper but in practice it just feels slightly more responsive than a rear-hub does. On cargo ebikes that's a $1000 difference and the real upgrade you get from that is a Bosch motor that will last 80k km. That's one of the things making me look at Tern GSD models ($3500-4000 used), but it isn't necessary at all and doesn't add to the experience.

Larger tyres might also be more efficient on paper, but you're trading off accessibility and safety. The motor means you're already going as fast as you can pedal on a normal bike. There's nothing to gain there from any tyre size. With a kid or cargo you want maximum stability and control over the bike. A small tyre makes it easy to stand at a stoplight, walk it over ice, and load/unload without tipping. Wide ones absorb a lot of shock, handle uneven sidewalks better, and keep you from slipping. I'd only buy a cargo bike with 20" tyres and don't notice any kind of performance gain from my commuter bike's 28" ones.

edit: Also with a kid you'll be going slower than the 32kmh class 2 limit of a cargo ebike or 45kmh class 3 limit that I don't consider bike components safe beyond. 24kmh is what I'd consider a safe speed for trail riding with a child or full cargo load because the most important factor there is your ability to stop safely. The level of exertion there is equivalent to casually pedaling at 8kmh on a normal bike.