Finding out Santa doesn't actually exist is a heartbreaking day in a child's life. I remember looking at my mother with betrayal when I discovered the truth, utterly crushed that the magic I had put my faith in wasn't real. The longer you believe, the worse the pain will be, but as a parent, you want your child to experience as many truly magical Christmas mornings as possible. Any honest adult will admit there's just something missing from the holiday after their kid finds out Santa isn't real, which is why they make sure there are things Santa leaves behind to prove he came down the chimney (or in some cases, through the front door).
The key to keeping your kids from getting suspicious about Santa is paying attention to the details. Anyone who tells you kids don't notice things have clearly never met a real child; they can tell if you substitute their favorite blanket with a lookalike while you wash theirs, so they'll definitely be on the lookout for evidence Santa really came to give them gifts, especially as they grow older. There will be some kid (or teacher) trying to tell everyone at school that Santa isn't real, and your little one will believe them if you don't go big with your commitment to the North Pole bit. To really convince your children about Santa, it's best to start early — send letters to Santa, don't forget to move the Elf on the Shelf, and you can even get your child a personalized video from Santa thanks to Portable North Pole.
But once you've laid that groundwork leading up to December 24, you have to follow through with clues your children can find on Christmas Day of St. Nick's visit. The presents signed from the jolly old man himself will obviously do some of the work for you, but a few small touches can really make all the difference. And if for some reason your kids are doubting St. Nick's existence, teach them my mom's favorite motto: if you don't believe, you don't receive.