Camping gears can be found at following places:
1) Mountain Equipment Co-op:
www.mec.ca
2) Bushtukah:
www.bushtukah.com
3) Trailhead:
www.trailhead.ca
4) Le Baron:
www.lebaron.ca
1, 2 and 3 are close to each other (near Westboro Loblaws). Mountain Equipment Co-op is a member only store. You pay $5 for permanent membership. Le Baron is a specialty outdoor store in Bell's Corner.
For camping after mid August and before mid September, you should expect night time temperature at around 5 degrees. A sleeping bag rated at 0 or lower temperature is necessary. Always bring a fleece blanket just in case.
For tents, you should get a "foot print" for your tent to separate the bottom from the ground. If your tent doesn't have a foot print, you can get small tarp instead. For 4 people, don't buy a 4 people tent because it'll just fit exactly 4 people. You should get a larger tent for your comfort. Eureka is the tent of my choice.
As previous post says, illumination is important for camping. Always bring at least two pieces of flashlight and back-up batteries. A propane lantern (or new battery lantern) is needed for night time activities.
Bring some ropes and big tarps to setup a shelter for your dining area and tents. It's useful if it rains.
Finally, check the area before setting up your tents. Make sure you clean up the tent site. Also check your camp site for any protential danger since you're bringing a kid with you. There may be some wild animals at night in Algonquin Park, so make sure your food is safe or they'll be all gone or ruined at night.
Btw, don't forget to clean up the site before leaving.