I am very uncomfortable giving out my SIN number to my daycare clients and have yet to be asked thankfully. That is my personal identity that I don't give out easy.
A BN is issued by the federal government and is used to track remittances for CPP, GST, etc.
A BIN is provincial and is a registration of your business and its name with the Provincial government. When you do this, you will get a Business Master Licence with your number on it and can use it for your income tax recites. It has an expiry date so keep an eye on that.
A fellow daycare provider used her sons to shovel snow etc so she employed them and needed a BN for federal remittances. The BN is the id for the CRA.
I guess it depends on your needs as to whether you need one or both.
It is very confusing!!!!