Clearing up the mix-up here. Children of Time is the first book in the series that centers on a spider civilization while Children of Ruin is about octopus.
I haven't read Children of Ruin yet, but I've consistently heard from people that they enjoyed Children of Time more.
I won't hype it to the moon. I enjoyed Children Of Ruin more, which centres on spiders. But it's a decent read if you like SF.