Assuming we land with them in Canada, what do we do
This is a bit of an artificial question. The likelihood that you would find out on the plane seems very low. Even if you do, you could probably just dispose of them in the baggage claim hall following the "safe disposal" process as much as is feasible. See Safe disposal of prescription drugs on the official website of the Government of Canada.
If that's somehow not an option: I generally don't condone illegal behavior but in this case the best course of action would be to "do nothing": Just walk out with the drugs and dispose them as soon as you can safely do so. The chances that you will be stopped in customs, searched, and that the drugs would be found and identified as illegal are VERY low (unless there are other things about you that might attract customs attention).
Even if it gets found, you have plausible deniability. You bought this as an-over-the-counter drug for personal use in small quantities, so it's not unreasonable for someone to assume that these are perfectly fine to bring.
Of course ignorance does not protect from punishment, you are breaking the law, and it's hard to predict what exactly will happen. Most likely outcome here is probably that they would simply confiscate the drugs and leave you off with a stern warning or a fine, but it's also possible that they decide subject you to legal action.
More legal musings on the page Drug Importing Into Canada.
One important aspect mentioned
The Crown must prove both the act of importation, and the knowledge that the accused knew the drug was a controlled drug or substance, as per the Schedules in the CDSA.
Ironically this makes it less advisable to disclose the drugs to customs. By disclosing you admit knowledge that the drug was a controlled substance. If you get caught while walking out, you can credibly claim that you had no idea.
CAVEAT: I'm not a lawyer, this is not legal advice.