The main thing I would say to potential readers is to judge this book on its own merits.
A number of reviewers compare "Child of Fire" to Jim Butcher's "Dresden Files" novels. Yes, this is Urban Fantasy. Yes, this is Dark Fantasy. Aside from that, I didn't find a lot of similarities. Keep in mind, just because Butcher likes a book, doesn't mean that it's the same kind of book that he writes!
Ray Lilly is in prison. We get some hints as to why, but I expect his back story to be revealed gradually in forthcoming books in the series. He is released from jail by the "Twenty Palace Society". We know even less about them, just that their mission is to track down and destroy anyone who is using magic. (Besides them, of course.) He is assigned to work with Annalise, and we don't know a whole lot about her either, other than that she is almost indestructible and has a really bad attitude about almost everything. Especially Ray. Who she fully expects to die very soon.
They are sent to a small town to investigate the disappearance of children, and the strange fact that once they disappear, no one remembers them. Not even their parents. The rest, as they say, is the story. And it's told very well. The narrative is tight and fast paced. The main characters aren't very likable, as some reviewers have mentioned. And on first appearance, they're not. But we don't know a lot about them, and they have a very unlikable job: killing anyone who has, or has been touched by, "unauthorized" magic. And that means there is frequently a lot of collateral damage.
This is obviously the start of a series. I'm willing to give Ray and Annalise the benefit of the doubt for now, until I know about them and their mysterious employer.
This is not Dresden Files, so don't expect a likable wizard, a sarcastic skull and a cute police woman. And don't expect much humor, unless it's the very, very dark kind. One reviewer compared the book to Lovecraft's Cthulhu Mythos. Sorry, I disagree completely. This book isn't much like the Mythos at all, except that they are both in the dark fantasy and horror genre.
This book is dark, grim, and frequently unpleasant. And withing those parameters, it's an excellent book. If this is the kind of story you like, then I recommend that you give it a try. But approach it without expectations. Don't expect Butcher, Lovecraft, King, Koontz, or anyone else. Just expect Harry Connolly, and let him take you into his own dark world.