If a daycare is "under the table" I am assuming that means the money they charge parents is not claimed on their taxes as income. And I am further assuming that if they aren't claiming it on their taxes, they aren't providing the receipts to the parents either.

Well, that's your 'hook' so to speak. You can give receipts for the parents to claim, the others can't. I'm sure the CRA would love to hear about these daycares.

If, you think they may be illegal, meaning operating with way too many kids, then I'm sure the Ministry would love to hear from you too. If you plan to make this your profession and you will act professionally, then you should expect it of others too.