All good suggestions for Ontario but you will need to travel through beautiful Manitoba first. You did not indicate your starting point so I will suggest a couple of different ideas. If you are on the Yellowhead Highway, it is worth a bit of a detour to visit and camp at Riding Mountain National Park.
If you are on the Trans-Canada Highway and depending where you are at each part of the day, I encourage you to consider:
-Oak Lake Island Resort which is a few miles south of the TCH just west of the town of Oak Lake. It is a great beach area for little kids, shallow warm water and all needs including ice cream.
-a bit east of Brandon is the Spruce Woods Provincial Park. It is a bit south of the TCH. Lovely and rustic.
-if you are close to Wpg when time to stop, Miller's Campground is a bit west of the City of Winnipeg and a nice family stop.
-if you are getting east of Winnipeg by nightfall, then plan to get to Whiteshell Provincial Park (Falcon Lake, West Hawk Lake)
This leads you into Ontario where Lake of the Woods area provides some lovely campsites around Kenora and onward.
Provincial and National Parks require a pass, of course. And, what you miss on the way out, you might catch on the way back!
Enjoy and may you be blessed with good weather and safe travel.