I just tell them to have a tax professional call me or the CRA can call me themselves. I have never had a penalty and if they even tried to enforce one I would fight it because its just not safe to give your SIN number to every person you work with. Just think about how many families over the years we work with and how many lost receipts that could add up too. Identity fraud is huge! I have no qualms giving it to Revenue Canada but I've had some real winner families who I would never trust in a million years to give my SIN number too. Who knows what those parents would do with it also. I keep ALL my paperwork for the year and totally ready if they were to ever question me. I've had a client who worked for CRA and she even told me not to give it to parents!