Beth Lewis structures the book in the mode that's popular with a certain type of novel of having each chapter from the point of view of one of a range of characters - in this case, three individuals. There's James, a newspaper reporter who was supposed to be spending just a week in the cult to write it up - but also has an ulterior motive. Then there's Eve, a former cult member who has left and now wants to find their secret location to take her revenge on its leader. And, finally, there's Root, a six-year-old child, one of a number of 'beams', young children brought up half-savage and fed only on a mysterious cake containing metal particles, who will apparently (and sinisterly) be involved in opening the golden door.
It's an impressive piece of writing. Apart from the various backstories, which can be a trifle tedious, both the life in the cult experienced by James and Root, and Eve's story - which is mostly that of an amateur detective, on the brink of running out of money, trying to find out who Sol really is and how to get to the cult's secret location before the eclipse - are highly engaging. There are some oddities in the stories. It's hard to believe James, who is terrified of everything, could ever have made it as a newspaper reporter, while Root has somehow picked up pretty well everything about English but how to conjugate verbs, making his voice feel strangely artificial. Eve's is the persona that is most believable.
However, though recommended to me as 'speculative fiction', which is usually code for 'science fiction that wants to be treated as literary', this is really science fantasy, the genre that combines scientific tropes with a fantasy set of rules, arguably including classics such as Roger Zelazny's Roadmarks, superheroes and the likes of Star Wars. Although there's a scientific starting point here in what seems like the many worlds hypothesis, it lacks the required degree of plausibility. Thanks to some sort of magic energy from the Sun, channeled with a combination of unlikely sounding technology and the beams (though later on it unexplainably works without the beams), we are asked to believe that Sol can somehow pinpoint an alternate universe where all the cult members' key bad decisions were never made, out of all the near-infinite alternates.
Perhaps least scientific is the idea that this magic power from the Sun can only be harnessed at the time of an eclipse - in fact Lewis even acknowledges this towards the end. Eclipses are emotionally powerful and convenient for the plot as they provide an immutable deadline - but they are scientifically trivial. All an eclipse could contribute is in blocking energy from the Sun, which runs counter to the whole idea. But even this isn't the most implausible aspect: that is Sol himself. He was apparently a child prodigy, winning the Fields Medal in his teens and becoming a leading theoretical physicist. The idea that an individual with the personality traits required to be a mathematical child prodigy could also be a charismatic leader of a cult seems extremely unlikely.
Nonetheless, I found Children of the Sun a compelling read - and it has a clever twist at the end. The cult aspect is powerfully described, and Eve's race to get to the site on time is nicely managed. Just don't expect speculative fiction in the sense of literary SF.