Because false beliefs that are later dashed make kids cynical? Being prepared to distinguish reality from fanatasy is an essential skill needed to rise above the whims of crowds. However, if I had a kid in a similar situation as the OP, I might have tried to teach them about the dangers of confronting other kids' parents when those kids stop believing in Santa at four instead of whatever age they'd hoped.

The premise is unfalsifiable, so claims of falsehood are unscientific. Doing this is just as bad as preaching from a book if you care about truth.

That's ridiculous. The claim that unicorns and hobgoblins don't exist is also unfalsifiable but we human beings aren't purely rational automatons that can't tell the difference between fantasy and reality.

In the absence of falsifiable claims, the burden of proof lies exclusively on the person making a positive statement of existence (that there is a supreme being). If you care about truth, preaching from a book is so much worse.

Do you always downvote people you disagree with?

At least those fictional characters aren't taught to kids with a threat of eternal pain and damnation if they do something Bad.

(Although in NL, Sinterklaas' helpers will abduct you to Spain if you're naughty)

(edit: Actually that doesn't sound too bad, I wouldn't mind going to spain)

Isn't one of Santa's big things that he has a list of good and bad children and you will receive coal instead of presents if you are bad? Lots of parents use Santa as a threat especially to misbehaving kids in the weeks leading up to Christmas. I don't think there's anything wrong with that but there is definitely a 'bad' side to Santa as their typically is with religion.

At least it's not Krampus, who'll drag you off in chains.

Kids don't choose to believe in any of that stuff. It's a game for adults that involves extensive lying and often elaborate staging. You're explicitly equivocating between cynical and realistic, here. Everything true is not bad, and the adult world is not all terrible.