First you will have an 11 hour drive to get there. Not including construction, slow traffic and rest stops.
We went to Cape Crocker when the kids were little and they really did not like it. Boredom was the word. They are and were very outdoor kids and enjoyed camping every weekend. We find that the northern part of Bruuce County and the peninsula is not child friendly unless you intend to stay exclusively in your campground. I hope someone has a more positive review for you.
The Bruce has many places to fish, don't forget you will need a license. If the trip is for your granddaughter then I would stay somewhere in the area of sauble beach. It is a little lower down than you wanted. The water is warmer there. The water on the peninsula can be very cold.
Wherever you go have fun with your granddaughter.