I agree with Photobear Sam that trying to do 3 provinces in 1 vacation may be asking too much. NB and PEI in one trip is great, if you can get a bit of NS and Cape Breton in you're doing well! Don't forget, some highways are only 2-lane so that will slow you down considerably, especially if you're entering NB from Quebec and are stuck on the horrible 2-lane until you get near Fredericton. If you cut through Maine and enter Canada at Calais, ME/St. Stephen, NB you'll be on a faster track, only about 3.5 hours from the border to Moncton. Also, PEI is entirely 2-lane with rural folks who don't hurry (expect tractors on the highway). Add a bunch of gotta-see-it-all tourists who are used to the 401 and ... you'd better keep your wits about you!
I've lived in southern NB, Cape Breton, NS and Halifax, NS and agree with what everyone had to say about those places. You likely won't get a chance to do this on your trip but I just want to add for anyone considering a trip to the Maritimes: the coastal drive from Halifax to Lunenburg, NS (by way of Mahone Bay) is FANTASTIC! If you're camping, don't miss the Ovens provincial park near Lunenburg - cliffside camping above sea caves that you can explore in Zodiac boats and even go inside. Go inside a cave at night in summer and sea it glowing green & purple from phosphorescent sea creatures! One caveat: cliffside camping means it's probably not the best campground if you have toddlers and I wouldn't put too much faith in the old wooden fence along the edge of the cliff either!
At Lunenburg, explore the Maritime Fisheries Museum - name sounds boring but the place rocks! Outdoors at the museum's wharf are 3 floating exhibits you can board: a fishing schooner called the Mary Theresa (cousin to Bluenose) that you can climb all over and sea how they lived, handled salted cod, etc. There's a modern fishing boat with a ship's bell that was lost at sea and drug up years later in a fishing net by the son of the captain that lost it! How's that for probability! And the best part is you can also board the Bluenose II if it's at anchor there (she's a travelling exhibit and sometimes visits Halifax, etc.) You can even take a short cruise on the Bluenose at Lunenburg (separate charge from your museum admission) and it's spectacular being out on the water on the Bluenose - we spotted the caves at the Ovens from the water!
Also, between Halifax and Lunenburg there's Peggy's Cove (a bit out of the way but worth it) and the most important advice I have is to WATCH YOUR KIDS around the rocks near the lighthouse and water - there are rogue waves here that occasionally sweep over the rocks. (In peak seasons they have lifeguards, but not always.) Also, there is the Swissair 111 monument further along St. Margaret's Bay that is nice to stop at if you're going that way.
Okay, this is getting pretty long so I'd better sign off! Have fun planning!