If you want to start piano theory for your daughter, you can check your daughter's school. There is a group piano program in school, one hour once a week. The price is pretty reasonable. Normally, the class is hold in Gym. It's good because the little kids are learing piano in a very familar environment and with familar people. The bad thing is that every kid only has around 5 minutes to practise.

You can also go to square one, there is a instructment store. They provide both group lesson and private lesson. My friend told me their program in pretty good.

If you want to choose private lesson, there are a lot of music school in mississauga. Get a mississauga new on Friday, check kid's page. You can see a lot of music school advertisement.

Good Luck