That's great Karen. The younger you start them, the more time they have to learn.
Although I don't have any kids to bring, having been a Sunday schools teacher, (yes even with my evil streak), how about somethign like a plant lore quest, see how many different trees or plants the kids can find/identify. The winner getting a bar of chocolate or something.
Rather than defoliate the place they could pick up a 'ticket' attached to the tree, that they write 'oak', 'ash', etc., on or give them a list of trees to find, number some trees (a piece of paper tied to a branch) and get them to match the numbers to the names on the sheet.
I'd be happy to help organise something like that. As Gary says in his signature
Knowledge is a gift the wise will freely share
.
I'll even stump for some home made chocolates/cakes or something.