When my girls were infants and couldn't write, they scribbled or drew a picture and I wrote the thank you note and put the picture in with it.
When they were in grade school they were not allowed to play with the gift until the note was written. They wrote a simple thank you on card and "got to mail it at the post office" which they thought was neat.
When they were in high school they new what was expected of them without even telling them.
They are both married now and always send thank you notes without even being told, even though I always remind them of it and I get a "yeeeess mom", thanks for reminding me.
Neither one have any children yet, but I am sure they will make a big deal out of the issue without me telling them.
By the way, I have gotten many calls telling me what nice and thoughtful notes my daughters have written them.
They continue to make me proud.