Many adults believe that kids best explore nature through their own vivid imaginations. Imagination is an astonishing and wonderful thing. But in the absence of real information, both kids and adults tend to ascribe human emotions and motivations to other living things. This does nothing to help us discover the interconnectedness of all life. We cannot simply rip the fabric of life apart to suit the convenience of humans and expect nature to be just as adaptable as we are. The notion that drilling in the arctic is fine because the caribou can just find some other place to go, is based on that kind of ignorance. A little knowledge will help you find the endangered wildflowers in the botanical preserve and the endangered butterfly that depends on them, the common edible weed that you can gather for dinner, and the invasive wildflower you can pick for a bouquet. It will help you discover which threatened frogs are breeding in a local pond, where you can root for their survival, and which ponds are full of invasive bullfrog tadpoles that would make great permanent pets. A little knowledge is useful.
My personal strategy is to leave most of the guides at home when we're out exploring. I don't want us staring at books or screens when we're outside, and I also don't want to worry about the extra weight or the devastating effects of leaky water bottles. Instead we bring cameras, get close up and take photos. Then we look up what we've seen at home. Here are brief reviews of the guidebooks that have been most helpful to us.
One City's Wilderness: Portland's Forest Park is the book to have on hand if you want to explore Forest Park. It's the only urban wilderness city park in the US, and it's full of beauty. But it's a bit intimidating to just dive in and wander, since you can't yet get a map that shows all the trails together, and trails within the park can be poorly marked. This book is the key to this piece of green Eden. It will guide you properly, and tell you just what to expect from different hikes in terms of terrain, distance, sights, etc. The author, Marcy Houle, leads guided hikes fairly often through the Forest Park Conservancy.
For quick, searchable databases that will help you track down thousands of great hikes, check Oregon Hikers and Washington Trails Association. There are so many, many guidebooks to great hikes in the Pacific Northwest, including quite a few that are specifically for hiking with children, that it's impossible to pick out favorites. My advice is to whip out your library card and get ready for a bonanza of good advice.
identifying tracks will often serve you well.
For a pocket sized guide, Scats and Tracks of the Pacific Coast by James Halfpenny is good, with the benefit of descriptions of scat; and Animal Tracks of Washington and Oregon by Ian Sheldon is also good, with the benefit that it includes excellent descriptions of each species. Bonus points for including sasquatch! Pocket guides to animal signs are great because these are the guides I find most worth bringing along on a hike. It's hard to remember just what clues I'll need to use to make a track ID, and my photos sometimes don't show all the important details. However both of these books ignore nutria. Hey, they may be invasive, but they are here to stay.
For reptiles, I love Reptiles of the Northwest by Alan St. John. Aside from the ubiquitous garter snake, which loves to chow on our plentiful amphibians, the Portland area doesn't provide a lot of excellent habitat for reptiles like Eastern Oregon does. So my book doesn't see a ton of use, but I'm always glad it's around when I need it.
The definitive guide to all things avian is the Sibley Guide to Birds, just revised in 2014, and the Sibley Guide to Bird Life and Behavior, both by David Allen Sibley. They are superbly illustrated and fun to read. Sibley's Birding Basics is an outstanding guide for anyone wanting to take an interest in birds a bit more seriously. If you're interested in learning to bird by ear, Bird Songs of the Pacific Northwest by Geoffrey A. Keller is a definitive audio guide. While it's intimidating to imagine learning the bazillion songs on these CDs, setting a goal of learning just a few each month is much more manageable.