When you take your kids to a museum where there are animal bones (skeletons), or stuffed animals, do you have these kind of discussions? It's too much for the kids. I don't think that you can pull off an objective discussion with this age. And they WILL put you on the spot. You have no idea what the families of these kids believe in regards to hunting. You don't want to paint the hunter in a bad light - you could essentially be making a child see their own dad or uncle or granddad as a bad guy and that's not really your place to do.
Instead of making it an opinionated study of the remains of the deer, make it a biological study. All children this age know that eventually an animal dies, so if they ask why this animal died, don't bring up hunting. Just make it a natural event.
Dawn