Prince Edward Island can be reached by the Confederation Bridge from New Brunswick or by ferry from Nova Scotia. The toll for either is paid when leaving PEI and the bridge toll is significantly cheaper. IMO if you are planning to visit Cape Breton first it would make sense to take the ferry to PEI and leave via the bridge.
The plan is 4 weeks (August) and 4 (5-9) day stops. PEI, Cape Breton, somewhere near Halifax, and then somewhere on the Fundy National Park/St. John side.
I was not thinking straight when I posted the part about PEI. I was thinking based on my situation and yours is different. When I go there I calculate my cost getting there from my location and back as a factor in the trip. Going for one week it cost me around $618 ($300 fuel and $107 ferry ride plus my camp site to make up the rest) Two weeks cost me $785 (my gas and ferry ride is the same and most camp grounds will give you 10% off min for the extended stay)
Based on your plans, I am assuming your coming from the West area and would take the bridge to PEI then take the boat from PEI back to the mainland Nova Scotia. The Ferry is $50 more then the Bridge but you save more then that in Fuel as it is roughly 6hrs of hauling between the point of getting off the bridge and to where the ferry drops you off in NS. Time saving is hit or miss as you may have to wait for the ferry.
My advice would be a week in PEI, off the ferry you are a 4 to 4.5hr drive to the Seal Island Bridge Camp Ground or any of the camping areas listed. When you leave Cape Breton if you stay in that area your 6hrs hauling to Halifax where you can set up for your week then move on to St John.
You could take the Bridge back and go to Halifax then work down to Cape Breton but not what I would do. I would find the trip back from Cape Breton to St John needing a night set up somewhere to break up the drive. The trip could be made in a day but I would be cranky lol.
I will keep a look for the summertime review that list a lot of the NS events and post a link when it comes out.
I live in Sydney so I'll try to help a bit. In Baddeck area we like Baddeck Cabot Trail Campground the best http://baddeckcabottrailcampground.com/ We have a couple young boys and find it to be the most family-oriented. It's no Marco Polo Land like in PEI, but still a nice quiet clean spot.
You definetly want to drive the Cabot Trail. It is best to do it in a couple days if you can. Or you could do Cheticamp side one day and then head back to Baddeck, then do the Ingonish side the next and head back to Baddeck. We have never stayed in Cheticamp, only Ingonish. Both spots have lots of sites. There are a number of hiking trails in the park. If your kids are young there are probably a half dozen they could do. If they are school aged or older than you should definetly do the Skyline Trail. It isn't too rough, I think it takes about 3 hours round so plan accordingly, but well worth it!
In the Sydney area I recommend Fortress of Louisbourg, the Giant Fiddle, and the Miner's Museum. A little known attraction that we love is Two Rivers Wildlife Park http://www.tworiverspark.ca/index0.php It's about 20 minutes outside Sydney.
If you are into hiking check out http://www.moosebait.com/ It lists a lot of the popular hikes. Good luck and have fun!