Reddit mentions of Crocodiles and Alligators

I don't know of anything great for kids on fossil crocodylians, but there are definitely kids' books on living alligators and a few on crocodiles. There are a few on Sarcosuchus like this and this They don't get the press that dinosaurs get.

A friend gave me this pop-up book as a gift and it has a couple crocodylomorphs in it (extinct non-crown-group crocs). That whole series seems to hit on fossil taxa that other books don't. The book itself is a little fragile but it's really gorgeous.

This book is a good basic reference on all the living species. It's not a kids' book, and it's advanced for a young child.