TD Meloche Monnex covers me for 6 kids, my own and 5 daycare children. They are covered in my vehicle (although I never have 6 in my vehicle) but on days that I have less than 6 we often do field trips and they have assured me that I am covered and do not need "paying passengers" insurance as it is not a commercial business and just a home based one.
I would try them. It's not super expensive and it is just a ryder to my home. Meaning they only cover for when a child or parent gets injured on your property, not any food allergies, abuse or neglect (which a commercial daycare insurance policy would cover)
And since I don't plan on abusing neglecting or serving them a food they are allergic too I feel I am covered pretty well
But that's just IMO