Have you considered visiting Cape Cod, since you are headed to Boston? It has some of the most beautiful beaches and sand dunes in the Northeast, as well as plenty too do, such as visiting lighthouses, sailing, hiking, biking, visiting aquariums, etc. Disney has a family guide and suggested itinerary here:

http://family.go.com/travel/vacations/massachusetts/cape--cod/best-cape-cod-vacation-for-children-282103/

Have a great trip!
Jeanie @ http://www.vacationdaydreams.com

Just one note - August is racing season in Saratoga and lodgings have been booked full for months - at double the usual prices. Lake George is also likely very full - for the same reason (Saratoga overflow)l I wouldn't count on finding places to stay in either one unless you already have reservations,

If you really want to do this I would search for something now - you might have to stay further up Lake George to find something - or go further south and stay in the Hudson Valley.

Portland vs Kennebunkport is apples and oranges. Portland is a fairly good sized city - a bit gritty, but a lot of fun. Kennebunkport is a quaint beach resort town with a nice beach. With 5 kids, I'd stay in one of the beachy type towns - Kennebunkport, Wells, Ogunquit or York. Do Portland as an afternoon excursion for dinner and shopping. There are several places right on a nice beach, like Lafayettes in Wells, and the Norseman in Ogunquit. Old Orchard Beach is 'ok', a bit honkey tonk but has a great beach and lots of amusements for the kids.
Your White Mountains plan sounds good - just choose a good day for the auto road.
You might want to spend more time in Vermont and pass Saratoga due to racing season. Vermont is extremely scenic, and the kids might like a few nights at a farm b&b like Liberty Hill Farm. Another fun place with kids is Billings Farm and Museum.

Mt Washington auto rd can be very difficult on your car's brakes on the way down. Kids might enjoy taking the Cog RR up instead. Agree with previous posters that Old Orchard Beach is very honky-tonk, skip Saratoga and stay in Kennebunkport/Ogunquit or other small town rather than Portland.

It's a shame you have been planning this trip since Feb and still have not locked down anything for next month, peak season.

I see no reason to stop in Boston for a hotel. Depending on your chosen route you could stop at Sturbridge Village (MA) for a visit and the night, or Mystic CT. Then I would head up to either York, Ogunquit or Wells the next morning. In York you have an animal/amusement park, a lighthouse, a taffy making place, ice cream, a great playground and a pretty active weekend entertainment series. Ogunquit, you have Perkins Cove, lobster boat trip, Marginal walk, a 3 mile long hard sand beach. Wells has another great beach and may be a bit more affordable for a home base, as you will need 2 rooms for 6-7 people I would imagine. Spend part of a day exploring Kennebunkport, Rachel Carson Sanctuary, Wells sanctuary. And let them be kids and play in the sand and surf.

I would blow off portland all together and choose either vermont or nh for the remainder of your stay. Even if they are great travelers that is way too much driving time in a packed car.

But that's just me. Good luck and get working because the best lodgings will already be booked.

sounds like a lot.. by indoor waer park do you mean coco key in danvers ma? you could drive right to portsmouth, nh, a beautiful and fun seaside town.. more quaint than portland.there is also an indoor waterpark in north conway, in the white mountains-kahuna laguna..both indoor water parks are in hotels..vermont is great, there is a cider factory near ben and jerrys, also cabot cheese. burlington is fun to do and lake champlain chocolate factory/store is on the way to vermont teddy bear factory. old orchard beach described as honkey tonk is being kind, I would skip it..drive route 1 from kittery (shopping outlets) through york and wells (beaches, shops, restaurants) to kennebunkport..the girls will love yummies candy store in kittery! good luck and have fun!

ps, check out santas village instead of storyland.. about 45 minutes further, but well worth it, nicer, cleaner, lots of shade..new little waterpark. if you enter after 3pm, you can go back the next day too.. and admission is about $25 a person. good reasonable food inthe park, refillable drinks..they have daily deals on their website.. my kids (3, 6, 8) love it! and dont care for storyland...