There are different types of field guides.
Considering the ages of your children, I recommend Fun With Nature as well as the second book called More Fun With Nature. My children greatly enjoy both books. In fact, we need a new copy of Fun With Nature because the binding has fallen apart.
We like the National Audubon Society Field Guides, too. Several of those are region-specific.
Sibley's Birding Basics is another well-worn favorite here. It shows depictions of the wings of birds in flight so you can learn to distinguish one bird from another when shape is your only clue to identification because of the sun.
I know we have a few more favorites, but they have apparently migrated off the shelves and up to the children's bedrooms. I'll have to wait until tomorrow to look for them. :)