Wait, I know how these “belief” movies work. The parents don’t believe, so that means that they are oblivious to the supernatural even when it’s right in front of them. So they just convince themselves that they bought the gifts.
Or maybe magic is involved. Obviously, Santa would not make people not believe, since he always seems to value those who believe. But maybe there’s a Satan to Santa’s Jesus, someone who can warp the minds of the unbelievers. The Anti-Santa wants everyone to forget about Santa, so he won’t exist. And, of course, that means that Christmas won’t exist, for some reason.