any guide book will do but Tom advocates letting nature be you rguide. Take children out for walks with your eyes open and some little bags in your back pack to collect stuff, field guides and a hand lense in the beginning. Sitting and listening and sniffing and watching are cool. Best to bring water and some kid friendly snacks (apples, gorp) if energy flags. It's amazing what you can see on your tummy in the grass on an (untreated) lawn.
There is time to read the book indoors if you need more ideas.