I'm not from Ontario, but there are lots of school closure snow days here in Manitoba. Unless the weather has affected my ability to provide care (like the power being out), my daycare is open and it is up to the parents to get them here. Even if school is closed for students, teachers will either still be expected to report to the school or will work from home - so I would not be surprised for them to send their kids. I worked as a teacher before and was expected to make a "reasonal attempt" to get to the school in bad weather when the schools were closed, unless the highways were closed.

I don't think you need to have any special wording in your contract. You just state that if daycare is open then fees must be paid.