We just returned from a family trip.

There is a nice, very scenic sandy beach, named "Sandy Beach," in Acadia, though the water is cold. The views from Cadillac mountain are spectacular, and you can drive to the summit. I agree that Acadia is the most scenic place on the east coast.

There is a very nice, very wide beach at Popham State Park, and, near Kennebunkport, there's Goose Rocks Beach, though parking requires a permit.

The harbor at Camden is well worth a visit, and, if you hike the upper trails at Camden Hills State Park (e.g., Mt. Battie, Megunticook, Adam's Lookout), you can get inspiring views of the harbor below.

We prefered the roadside lobster pounds to the sit down restaurants we tried.

Instead of a night in Portsmouth and a night in Portland, I would spend two nights in Ogunquit or Wells Beach or Kennebunkport. Movie from city to city the kids won't have time to enjoy much. 2 nights near the beach would be much more enjoyable, and Ogunquit is only a half hour or so further than Portsmouth.

What dates do you need something near Bar Harbor?

For Camden consider the high tide Inn, not sure if they do one-nighters in their cabins but the boys will love it.

Monhegan is not really a place for kids. If you're booked, don't cancel, but when I went and I'm no kid, I was bored silly after about three hours. Artistic types it's good for, but.....
The boat ride was the most fun.
We stayed at the Cedar Crest Motel in Camden last week and they have nice clean rooms , a decent pool,a restaurant good for families and reasonable rates. You will have to drive to the town center but if you get one of the free town maps, it's an easy short drive (less than 5 mins)and a creative look will easily find free parking. Waterfront Restaurant is excellent...no reservations, just get there by 6:00 pm. Terrific views.
Boothbay is great for one or two nights. You will see everything available for kids entertainment as you drive into the town and a sailboat ride from the town dock on the Northwind is a winner. The Tugboat Inn is right on the harbor and in town, so it's worth checking them for availability though a number of other places like Browns Wharf, Captain Fish's etc. would be ok for people who don't mind a short walk over the footbridge. I like Robinsons Wharf (short drive toward Southport) for boiled lobster dinners on the deck though most will say the Lobstermans Co-op right near the footbridge is better. No beaches here..different kind of place.
Acadia Park area..
We just stayed at the Bar Harbor Inn last week. Very nice place and a great location but unlikely you can get in without more advanced reservations and it is not inexpensive. Their package deal getaways are really reasonable however considering what you get for the price.

Just want to get back to everyone and tell them that we had an outstanding trip to Maine. Also want to give some feedback. We decided to keep our plans and to go to Monhegan Island. Turns out that this was probably the highlight of our trip!!! It is definitely appropriate for children (there were many other children there). We stayed at Monhegan House which had an oversized double that nicely accommodated our family. It was so wonderful to stay somewhere where you don't have to lock your door!!! And after the tourists left for the day, it was so peaceful. We did lots of hiking which the boys enjoyed. We ate some wonderful lobster while the boys skipped rocks... it was very relaxing. We would definitely go back; but this time we would stay longer.

We also loved Acadia. We ended up renting a house just across the bridge in Trenton. It turned out to be a great location. The house was on 4 acres of waterfront property which my boys loved (again, lots of skipping rocks).

All in all we had a great time. Thanks.

Your kids will love Monhegan. Just the boat ride over is an adventure. We saw seals and eagles.
The little cafe right by the ferry dock has wonderful chowder and sandwiches but it kind of expensive. You might want to pack along a lunch and have a picnic somewhere looking over the ocean.
In Port Clyde we loved having dinner at the Dip Net as we returned from our day trip to Monhegan.