I have brought my parents with us to have summer vacation in Maine more than once, they really enjoyed the trip.
You may also consider Acadia Natioanl Park. The trails in the park are fairly moderate, which are suitable for 7 year old. If your parents are in good shape, they may give it a try too. There are trails along the beach as well, which give the fantastic view of the ocean.
Bar Harbor doesn't have many big resort, but has tons of B&Bs. There are very family friendly if you travel with both kids and parents.
