With such a little one along I would stay far away from big lakes, due to wind that can pick up, as well unforeseen happenings with the kids or the weather (and the need to maybe leave early) Being by yourself and all the things you have to do, you'll have you work cut out for you just keeping an eye on them. Since you'll have no guarantee as to how far you have to paddle to get a site on opeongo, I would stick to the more reasonable distances of the canisbay sites. I've done many trips by myself with my children when they were little, and found the more controlled trip gave me more security ( to know I could get to the car and wasn't far from help) as well as the ability to enjoy the trip myself.
There maybe more annoyance with other families close by, but I would put up with that if it's only for a night or at most 2. And, a site with a bit of sand if not a beach, might help to entertain the kids. Nothing entertains them better then the camping pots and pans, a plastic shovel and hopefully some small creatures to catch on the waters edge. Always have them wear their pfd's and a foot covering like a sandal so they don't hurt their feet. No going between you and an eventual fire, and you take off any s'mores of sticks so they don't burn themselves.
Other then that, have a great time, enjoy them and yourself, and thirty years from now they may want to take YOU for a trip. It happened to me.