It's a good thing to learn, actually. No one is infallible, not even parents. And what is this horrible lie? What motivates it? A desire to create a little magic for children.
I was ten when I asked my mother if Santa was real, she explained it very well- that Saint Nicholas was a real person, but Santa is not and the reason why people let kids believe in him is to give happiness. I wasn't mad at her, my trust wasn't violated. I didn't feel "lied to". Even then, I was grateful to have been told to believe. I was entrusted to keep the magic going by not telling my younger sisters about Santa. My mother told me that in a way, Santa is real- because people are Santa. Just like in the "Yes, Virginia, there is a Santa Claus."
Santa embodies the spirit of generosity and love. People who donate toys to the less fortunate are Santa, as are people who pack boxes for Operation Christmas Child.
Santa is an allegory for unconditional love.
As per the op, people respond differently to the lies they hear, especially from whom it comes.
The PARENT may think a white lie is harmless (even though according to God, a lie is a lie is a sin.) But, that isn't always the case at all. The OP went through several philosophical and spiritual shifts because of the lie the parents told. I, with hindsight, would have been a confused, spiritual derelict if I had allowed my parents' lie to continue - it would show me the depths of deception they would go to just to create an
illusion they believe to be beneficial.
The fact adults go through so much trouble to justify something - even as a Christian practice - just shows the depths of deception of which they are capable. Especially, considering the stories of attaching the "reality" with the "magic" of Christmas - and none of it is in the bible.
Children aren't dumb; they are
innocent - just like Adam and Eve. They easily pick up on things for which adults think they have an intellectual authority - like deception, and breach of trust.
Eventually, the children will figure out the "magic" of Christmas, and Christmas itself likely, is a farcical lie. Then, they wi begin to think, "why would my parents lie about this?"
"What else have they lied to me about?"
We think "small lies" cause "small problems." Lies create problems period. I can see why God would treat even small/all sin with big punishment/testing.
We can't see that far into the future, but what if the white lie of Santa causes an intergalactic war between the Galactic Alliance of Santa, Santa's Invictus Nation, and the Truth of Santa's Kingdom 40,000 years into the future - destroying 1/3 of it? It sounds fantastic, but the fact is we don't know what impact our sin has on the entire creation.
One lie caused the entirety of the human race to lose our God given dominion of this planet, our God-given raiment and glory of light (specul.), and our perfection and pure connection of communion with our Father through Christ literally walking with us on earth. One lie.