I'll think on it to see if I come up with something...but while I do check out
Project Learning Tree (plt.org)
Project Wild (projectwild.org)
Project Wet (projectwet.org)
North American Association for Environmental Education (Naaee.org)
On my personal webpage I have links to various environmental groups
www.geocities.com/jdssquirrel/egroups.html
Hope that helps.
Some quick thoughts would be one of the sea shells talking about life in the sea as far as other creatures that they see. Keep in mind many of your group will be very familiar with Finding Nemo...use that to your advantage. The shell could talk about meeting a sea turtle, a clown fish, jellies, etc. It could also talk about when it washed up on the beach and what the beach was like.
Don't try to teach a strong environmental theme, they're not emotionally ready for that. Work on doing something that has to do with empathy. Winnie the Pooh is a great book for that age group because they live in the woods so therefore it can teach some basic environmental stuff about the forest being a home for creatures...but it also teaches empathy because little kids have things they are scared of just like Pooh. They also have favorite foods that they want always.