"Santa is real, but not in the way you think. 'Santa' is a guise-by-convention, a nom de guerre if you will, which we assume in order to do good without being named as individuals. There's no one living at the North Pole, but if anyone tries to tell you that there isn't any Santa Claus, they are mistaken."
100% true, and if they are old enough to understand the words they are old enough to understand the sentiment. Unlike with, say, the Easter Bunny, I'm sort of uncomfortable around people who tell kids there isn't any Santa Claus because it isn't really true. It's the truth but not the whole truth.
(Shout-out to Jim Butcher and Cold Days: it's a mantle!)
If I esteem mankind to be in error, shall I bear them down? No. I will lift them up, and in their own way too, if I cannot persuade them my way is better; and I will not seek to compel any man to believe as I do, only by the force of reasoning, for truth will cut its own way.
"Thou shalt love thy wife with all thy heart, and shalt cleave unto her and none else."