I agree with others here -- the best way to get kids comfortable in the woods is to give them plenty of time in the woods. Hiking, camping, playing, exploring. I just got back from a three days trip with another family, and the four 2-5 yr olds along were all pretty adept at pointing out poisonous plants, finding edible ones, knowing what grows where, scrambling rocks, helping start the fire, crossing small creeks, etc... I didn't have to manufacture any teaching moments, but they certainly happen, like when my 5 yo very excitedly showed me a different kind of fern he'd never seen before and asked about it. Mostly they climbed and splashed and ate berries and played. All these kids just know a lot because they get out a lot, and the most formal thing they've experienced in the outdoors is a regular weekly hike group (the sole formality of which is that kids meet at time X every week, and we walk somewhere, and they play).
I guess I do teach kids poisonous plants though. And basic fire safety (don't swing burning sticks into your brother, don't play balance games on the fire-ring rocks, etc...).