In your case, a Tern HSD/GSD longtail may be a better option. It’s not quite as convenient as a long John, but much more compact and parks upright, so it fits even a small apartment.

Transport capacity is still great and the bigger version can seat two passengers, but the kids need to be old enough to sit and hold on themselves.

I got an e-Muli, which is a sorta “short john” but where the front basket folds and handlebars pivot 90° so parking it is a bit more slim, more or less the footprint of a normal bike. It’s amazing — a little heavy so probably not luggable up stairs, but small enough when basket is folded that I prefer it even on errands where I don’t need the cargo space, just cause it’s electric (nice to throw a backpack in the front basket and close it, too. A little pricey since it’s made in Germany, but absolutely worth it: https://muli-cycles.de/en

London core requires a special solution -> bikes with folding handlebar stem, folding pedals, possibly a longtail if you need to transport two kids.

All of this stuff exists but the bike industry can't put 2 and 2 together.

Here is a link to a folding handlebar stem that is sometimes in stock: https://flatbike.com/product/thinstem/

I cannot speak for the safe part, but here in Belgium plenty of people leave their Urban Arrows outside (with a rain cover).

ive seen recently a new model of cargo bike that can be folded and it takes much less space. Myabe this helps