What's the Name of That Book??? discussion

So I probably read this book at least 10 years ago.
I don't remember a specific date and I don't recall how old the book was when I read it, unfortunately. I don't think it's particularly old.
The book starts off in a (dystopian?) city. I vaguely recall the city being described as laid out in a giant circle.
I don't remember much else about that, and the next part I can remember is the brother and sister (who I think have some kind of mental link?) fleeing through the desert. They get picked up by a giant wooden sand ship with wheels and massive spinning blades. The blades get explained when another giant sand ship arrives and they battle.
Some more crazy stuff happens and the next thing I recall is that they are fleeing an endless army of children. The children are all dressed impeccably and look almost "perfect". But they have no emotions and no thoughts besides killing. I believe there was a chant/song that they sang as they marched in a collumn. "Kill kill kill kill kill, kill kill kill."
The endless army of children zombies pursue them relentlessly until they flee over a chasm. The children keep coming and end up walking directly into the chasm with nary a scream, to die at the bottom. Thinking themselves safe, the protagonists sleep, only to wake up in the morning and find that the pile of bodies at the bottom of the chasm is starting to get high enough that the army of children who are still falling in to their death will eventually be able to walk over their dead compatriots to get to the protagonists.

The protagonists flee again.

It eventually ends up with the protagonists leading their family over the mountains to a "promised land". They reach the very top of the mountain to find that there is a sheer drop of many many miles to the ground. They eventually decide to jump down and are carried down by (wing? magic?) and live happily in utopia.

I remember thinking as I read this book that it was weird, so if this description leaves you feeling confused, I apologise.
Any help would be greatly appreciated.

I also just recalled that the male protagonist had an unusual name like "Falcon" or some such.

A Crack in the Sky by Mark Peter Hughes?

Much of this description reminds me of The Wind Singer by William Nicholson. The female protagonist's name is Kestrel, which would be the bird name, and there is an "infinite" army that chants "Kill kill kill kill" as well as a land sailing vehicle the main characters travel in for part of the book. Some of the details don't exactly match what I remember (they return to their home town which becomes happier rather than finding new utopia, for example), but enough do that it's a good possibility of being your book.

Yes! I think that's right. Thank you so much.