As for what people teach their kids about (insert fictional character) well, there's nothing you can do about that, so I'd prepare for the fallout rather than try and tell people it's wrong.
See if my DS overheard him singing Santa's not real he would probably a ) not care or b) be concerned that the little boy won't get any presents from Santa as only kids who believe get presents from Santa !
I love the fact that everyone has different beliefs. , DS thinks Jesus is a story the same as Rudolph the red nose reindeer is a story , I would hate him to have such a low self esteem that he would be upset if someone told him they believed something different to him
I think the lady in the shop is an idiot , mainly for having a go at a 5 year old and OP , thankfully we live in a country where your son can say what he likes and if other people are too precious about it that's their problem not yours !
ETA: As to what gets taught by parents to their kids, that's got nothing to do with me, unless it were to directly and negatively impact the belief my kids have. But I'm one of those people who believe that belief is important, regardless of what you believe in, whether it's science or God/The Gods or Santa Clause and the Tooth fairy.
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