How old should they be? What is if they have younger siblings? What if they straight out ask you if he's real? But no matter how carefully you try to approach this sensitive topic, it doesn't always go exactly to plan.
We've scoured Reddit to find the most hilarious, honest and sometimes brutal ways children found out from their parents that Santa wasn't real - it's probably best not to take too many tips from these families...
1. 'I simply noticed that the presents from Santa had the same handwriting as the presents from mom and dad. I was 5 or 6. To be sneakier with my kids, my wife writes the tags from us and I write the ones from Santa, and I write them in a fancy stylized printing to better disguise it.'
2. 'When I walked in on my Dad eating the cookies I left for Santa and putting together my bicycle. Sad and happy day. Dad and I finished the cookies together then I helped him put the bike together.'
3. 'I lost a tooth and decided not to tell my parents, just to see if the tooth fairy was actually real. When she didn't come the next morning I was pretty upset. So that night I told my parents what I'd done, and made them tell me everything they'd lied to me about, including Santa. Cue mom crying and dad laughing. In my mom's words I lost my childhood that night.'
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4. 'When I left a note asking for a piece off his beard (pretty weird thinking about it now), and the next morning I saw that the chunk of hair had been cut off of my dog. . .'
5. 'At age 3-4, I told my very religious mother that I thought Santa was better than God because he knew everything and gave me presents. She corrected that misconception real quick, and I went on to ruin it for all of my friends.'
6. 'I hid a camera in the tree and recorded my mom putting the presents under it. We watched it Christmas morning to my parents shock and awe, I was 7.'
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7. 'Older brother woke me up and pointed out the window into the night, and said "Look! There's Santa Claus". I snapped up looked out, eyes wide, my nose pressed against the glass, to see my dad unloading the car of presents. He said "dad is Santa. There's no Santa, it's dad. So be sure to thank him and not some make believe guy. Dad works hard."
'Changed my whole mindset. In my mind dad was mean. Unapproachable. He didn't even like me. But there he was out in the cold unloading boxes of gifts for me. knowing he wouldn't even get credit for it. Blew my little kid mind.'
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8. 'My mom sat me down to break this news to me around 7? I dunno. But THEN... decided to also throw in the "you're a girl and this is what a period is and where babies come from" talk. I cried all day and I was so sad.'
9. 'My dad gave me a science lecture on how it physically impossible for Santa to exist at like the age of 2. Also said if he was then he would be a criminal as he was breaking and entering'.
10. 'I found the box my trampoline was packed in just sitting in my backyard Christmas morning. Lazy parenting right there. They just said "Yeah, Santa is just us, sorry. But at least you still got a trampoline." Can't argue with that logic.'
11. 'My twin sister and I found all of our letters to Santa in a box in the attic. When we realized they never made it to the North Pole, we both started crying. :('
Image: Warner Bros. Pictures via Giphy
12. 'My mum always maintained that Santa could never reach all the children in the world because there were just too many! So she has forever been one of Santa's helpers. I have loved this theory since forever.'
13. 'I was four years old. I was appalled at the thought that some stranger could break into people's homes, eat their food, and leave some gifts. I asked my mom point blank... "Santa isn't real, is he?" She didn't lie to me, I was right. And that's how I knew I was a super sleuth.'
14. 'I saw my mom put my Xbox 360 under the Christmas Tree in '05. I let them think I still believed until I was 13 in order to keep getting big Christmas gifts as payback for lying'.