I have to agree with Heather & would highly recommend Answers in Genesis. They have a kids section on their website that has an animal of the week. Our sons, ages 8 and 11, think Answers in Genesis totally rock! We have used so many Answers in Genesis resources with our boys from a very early age. I can't remember specifically if they have field guides as you were asking. I have seen some pocket guides in their bookstore but can't remember the content. I would be interested in checking it out myself. We have the Zoo book that Heather mentioned & love it. I would recommend checking their bookstore where you can look for books, DVDs in a specific age group. They have lots of good book for education and leisure reading. I also would recommend anything from Buddy Davis on that site. We have a 2 cassette tape set of lots of fun creation songs that Buddy Davis sings. Again the boys love it. They still sing those songs.
If any of you live close enough to the Creation Museum near Cincinnati, OH and are interested in a great family field trip or just a great family outing, the Creation Museum is AWESOME!!! My boys say it is better than Disney World. It is absolutely breath-taking. It is a must see! 
Looking forward to hearing more. Hope this helps.
Angie