It depends on your province.
http://www.hrsdc.gc.ca/eng/workplaceskills/foreign_workers/caregiver/wages.shtml
There's also information about rest periods. If you're able to have a rest because there are no children at home or awake, some provinces allow a deduction for a rest period. This should be in your contract as well. In the event you're unable to take a rest period, they shouldn't be deducting anything.