I always give receipts but I notice that couple of the parents write on the memo line of their cheques - daycare fee for period so and so - which I guess is a precaution in case receipts werent provided. But Giraffe is right childcare expenses can be claimed without the receipt just make sure you have records of payment for when Rev Canada request proof.
I have never given out my sin # on my receipts to the parents but one parent got audited after 5 years of doing this and she asked for it, I gave it to her no problem. Just politely asked her to disposed of this information securely as soon as she gives it to her Accountant.