OK all well and good providing time is not of the essence as by the time all of the authorities have been notified they may have already entered Canada or some other country.

Parent picks up child at 9am as per custody agreement and has to have child back by 9pm.
In that 12 hours they could be in any country in the world and the other parent has no idea the child has been abducted or where they are.
Sure its not a perfect system but we do what we can.

Understood. But how would a consent letter prevent this from happening?

Even those travelling with a consent letter doesnt mean we won't check with the other parent if we suspect all is not in order. I have made hundreds of phone calls to the other parent even with a letter of consent. Our powers are limited as if the child and parent are Canadians or PRs or Status Indians then by law they have a right of entry into Canada and I have no legal grounds to arrest or detain unless there is a warrant that allows me to do so or they are committing an offence such as importing drugs or prohibited firearms etc.
Foreign nationals are a different matter.
The system is not 100%.

That is the reason why it is suggested to use a notary.

Myself and the mrs. have traveled by ourselves with our son more times than I can count. We have notarized letters of permission that we take with us but have never been asked going into or out of countries all across the world (France, US, Canada, Finland, UK, Japan and probably some I've missed) to present them. And that includes my wife coming into Canada with the young 'un on a one way ticket.

But you are carrying one and that is the main thing regardless if asked for it or not. Would you prefer if asked one day for such a letter a quick 2 minute interview while the officer looks over the letter or several hours in an Immigration Office while checks are made as you have no right of entry into that country?
I know which one Id prefer.

A cousin from back home visited Canada last summer with her daughter. The husband did not come. No questions asked as far as I'm aware.