I have run summer camps before but not in the manner you are thinking as I worked out of a licensed centre at the time ~ so unfortunately have no clue how you do this without being associated with a program that already has 'insurance' and so forth cause every time I did it the program while not a part of our 'license' it was part of the licensed centre I worked out of with their gym space as the 'home base' should there be inclement weather and so forth.

I know we were allowed to have a 1:10 ratio for children 4 and up and a 1:15 ratio if everyone was over 6 and up. If we were doing 'off site' field trips like to the parks, swimming pools, taking public transit and so forth we had to have a 1:5 ratio with volunteers who were over 16 (if I remember correctly that was an insurance requirement for liability reasons) ... other than that we were free to do with the program as we desired we spent most of the program time outside unless it was raining ... parents provided all the food in brown bags for 2 snacks and lunch and so forth.

My advice in Ontario is to call the Ministry of Education and inquire what the rules are around 'unregulated summer camps' and than call your insurance broker and ask about that as well.