Check out the Bar Harbor Oceanside KOA. The Western Bay is very calm (for the Atlantic) and you can launch from the rocky beach - or even your waterside campsite. Lots of activities for kids.
How old are your kids? I have paddled this bay many times and you have to be very cautious due to quick weather changes and winds. Sticking to the shoreline and not venturing to far out in the bay is best. It's a great place to paddle in good, calm weather as there are often seals in the bay and there are multiple places to "pull over" and play on the beach. Bar Harbor KOA Oceanside has the most fantastic sunsets, has lots of activities for the kids, is clean and well-run. Sites are a little cozy, but it's worth it. Due to the crowds, wildlife sightings in Acadia are actually somewhat rare, although if you go down to the horse stables at dusk you often see deer. There are seal and whale watches which give you a great great experience with ocean wildlife. If you get tired of paddling there are plenty of "tourist" things to do like mini golf, go-karts, and shopping.