I think the idea of the santa claus paradox is you just IMAGINE the sentence is true.
When you do that, you come to the conclusion that IF the sentence is true, Santa Claus exists.
Do you disagree that "IF the sentence is true, Santa Claus exists."
Now, read the sentence again: all it does is claim the very thing you've just agreed to, namely that if the sentence is true, Santa Claus exists.
You can't claim the sentence is false, as you know the sentence is correct when it says "If this sentence is true, Santa Claus exists".
To claim the sentence is false is to claim it is possible for the the sentence to be true, but Santa Claus not to exist.
Once you've agreed the sentence must be true, as it must, then Santa CLaus clearly must exist