You have picked a nice place to go. Lots of driving around - can hike and go to Cypress Lake as well. Remember to eat the fried White Fish. mmmm... There was a restaurnat in a Motel or Inn by the water ( used to be called River view or something, do not know if they have changed name. Go there and eat their white fish if it is still there.
I have been to Flower Pot island park. Not bad but a little bit disapppointed. I wonder if you might consider driving to Manitoulin lisland go up to Sudbury visit science north with the kids and come back through highway 69 ( 61?) anyway by land you have a nice time doing cruise on Georgian Bay - that is an alternative to just staying at the Bruce peninsula if the weather is too cold for camping. You may want to visit Bracebridge along the way and take the cruise of the Muskoka Lakes and visit Santa's World - the kids will love it.