We spent two weeks in Maine a couple of years back with our twin boys and another family with young children. We documented the trip extensively on our blog. Here is the first post which is a review of Camden Hills State Park, which we loved. I would definitely book early if you decided to stay there. There are many other posts and a review of the Bar Harbor KOA if you scroll up from the first Maine post. There are many great ideas in the blog for kid friendly activities. If you have any questions about what we did or where we stayed please ask.
http://livelylittlecampers.blogspot.com/2011/07/campground-review-camden-hills-state.html