If you provide your details with a donation, then your details get shared among other charities' canvasing arms. If you are prepared to donate to one cause than you may be prepared to do the same for others too.
Door knockers are not interested in one off donations, they only want subscriptions; that is how they make their commissions. They are immune to "Do not knock" stickers too.
One good thing about COVID is - no backpackers going around door knocking
RE What about the children type of letters - We had very bad experience with one of the charities "supporting cancer research" at Newcastle university (not part of University it self). They used to send letters disguised as greeting cards around Christmas time as many other charities do. The letter was to the effect of - what you feel about enjoying yourself when so and so with various misfortune are suffering - what do you have to say to so and so...
As usual, there are on their way to recycle bin but. unfortunately, our kid who was learning to read at that time got hold of it and started reading it.
Long story short - I did get our details removed from charities databases. No more addressed canvassing letters.
Regards,
George V