On the childcare deduction form that parents fill out for their income taxes, it does ask for either the childcare organization or the name and SIN for the childcare provider. Maybe this is why they are asking for it? Many people just write the name of their childcare provider and don't include the SIN and no questions are ever asked. But I suppose if they were audited, they would need this info.

http://www.cra-arc.gc.ca/E/pbg/tf/t778/t778-15e.pdf