Umm, regarding weight, we have Walk Assist which kind of negates the issue of having to push the bike up the stairs. The bike kinda goes by itself. I don't know if Bafang has it though, but since it has throttle controls, maybe you can just set it to go slowly. Not the same as having a sensor guide you, but should work.

As for derailleurs, my experience is it gets unhinged, starts rattling, you need to watch for different wear & tear on the different sprockets. Basically, it has multiple points of failure. Another issue is that on a kids ebike, an adult-size derailleur almost reaches the ground -- the clearance is far too low. So I would avoid. It's much easier to replace an IGH once every 10 years than to replace the cassette once a year or so. Really, the fact there are fewer exposed parts means much greater reliability. Also, since the chain doesn't get deformed on a gear hub, your chain lives 4 to 6 times (!!!) longer. I think this is significant.

What else? Oh, the chain doesn't bounce so you don't have to protect the frame.

You can check out this page, which outlines all the benefits of IGH over derailleurs... and yeah, some disadvantages, too. TL;DR is that IGH is an epic win.