OP has it confused. They lay out the plot to Children of Time which is the first book but call it Children of Ruin. Ruin was the second book that focuses on something else (no spoilers). Time focuses on the spiders.
I don't know why Children of Ruin came to my mind instead of Children of Time, but I agreed that the first book is more focused in the spiders. I was told that there is already a third book that end that serie, Children of Memory, gotta read this one! ...