The recent mentions of children calling parents' friends "aunt" and "uncle" reminded of this linguistic enigma. For all the complexities of English, we only have the one word for "you" (or archaic words such as "thou"). But in French, the word "tu" is used for speaking to a child, animal, or somebody you know well; for all other situations the more formal "vous" is used (also for plural). Likewise in German, "du" is familiar, "Sie" is formal.
When learning French and German, I remember being surprised at the way some of these were used. Although primary teachers would use "tu" for a pupil of theirs, a teenager would often be addressed as "vous", especially when being told off. I was amused to see that in the translations of the Mr Men books, they all used "vous" for each other. Children would mostly call their parents "tu", other relatives were more of a grey area. I was instructed to use "vous" with my great aunt.
My parents once visited our au pair's family in Germany, and were astonished when right from the start they used "du"; my parents weren't sure if they were supposed to do the same!
Someone once asked in a French lesson "if someone's mugging you for your trainers, would you call them 'tu' or 'vous'?", noting that criminals often called their victims "tu". Also, doing a quick browse on the internet, I was reminded that "tu" is used for talking to God. 
Not sure about Santa though.
Do you have any thoughts or stories on this? If we had the separate pronouns, perhaps children wouldn't need the "aunt" and "uncle" thing.