What to include, what to avoid, and three templates you can copy tonight.

Let's make some Christmas magic happen. I handle a great deal of the North Pole's outgoing post, so I have opinions about what makes a letter from Santa convincing and what gives it away instantly.
The difference is almost never the paper. It is the detail.
Include something Santa could not plausibly have guessed.
A letter saying you have been a very good boy this year could have been written by anyone. A letter mentioning that he noticed how patient they were when their sister was unwell in March could not. That is the entire trick, and everything else is decoration.
One specific detail beats three paragraphs of warmth.
Perfect handwriting. Modern printer paper. Being too formal. Promising specific presents. Sounding like a school report. And, most often, mentioning something only a parent in that house would frame that particular way.
Write it as though he is fond of them and slightly busy, because that is the register children expect.
Dear [name], Ho ho ho! I have been checking my list and your name is on the very good page. I heard you helped tidy up without being asked, and that is exactly the sort of thing my elves tell me about. The reindeer are eating far too many carrots and Mrs. Claus says they are getting round. Be good, get to sleep early on Christmas Eve, and I will see you soon. Love, Santa.
Dear [name], I wanted to write before things get busy. Jingleberry tells me the workshop is behind schedule, which happens every year and every year he says it will not. I have been thinking about you this year. It was not the easiest one, and you kept going anyway, and that counts for more on my list than most people realise. I saw what you asked for. I cannot make promises, but I have made a note. Look after your family this Christmas. They are lucky to have you. Ho ho ho, Santa.
Older children respond to being treated as capable rather than praised for being sweet.
Dear [name], Your letter arrived safely, and I read it twice. You asked about [their question] and the answer is [answer]. Not many children think to ask that. I have put your list somewhere safe. I cannot say what will be under the tree, but I can say I read every word. Thank you for the drawing, it is on the wall in my office. Love, Santa.
Answering their actual question is what makes this one land. Children remember being taken seriously.
Letters work beautifully up to about seven. After that, children who are starting to check will notice a letter is a thing a parent could have made.
What they cannot explain away is Santa saying their name out loud, and mentioning the thing from March himself.
See how a personalised message from Santa works.
One specific detail, imperfect handwriting, and no promises. Get those three right and the letter will be kept in a drawer for twenty years.
I have seen a great many of them, and it is always the specific line the child remembers.