It's been awhile but we did the magic triangle years ago, my wife and twins , age 7 at time. Took us two weeks. Started around Alexandria Bay, down the St. Lawrence to Montreal. UP the Ottawa river to Ottawa. Down the Rideau waterway to Kingston Ontario then back to Alexandria bay. Slept on 26' aft cabin Doral.
Google "thousand islands things to see". Boldt Castle was one memorable stop as well as just cruising all the little islands.
Things have changed regarding entering and leaving canada. Not sure how it works when cruising in and out of canadian waters. Typically here in Ohio, when going over into canadian waters to fish on Lake Erie, we just call an 800 number. they will want passport number, expiration date and driver's license. Once you are in the system, typically you won't have to repeat that info every time. Unsure on kids.
Most likely you will be required to report back into US customs at some point upon conclusion of your trip but , again, maybe someone local can give you a more definitive answer. We aren't required to report into US customs as long as we don't touch ground in canada.