"What would be your response if your child's teacher told your children that you are a liar and that there is a god? "
I'm too old to have schoolaged children. People are forever teaching things to children that conflict with what their parents tell them. Teachers, aunts, uncles, lots of people.
Parents hear about this stuff, and deal with it themselves, in their own way.
The fact is that Santa Claus is a myth, and should be taught as one. It's like all fairy tales. We tell them to our children, but we help them to understand that things like elves and fairies and unicorns are not real creatures but "pretend" creatures. Santa, too, is a "pretend" person.
Folks who believe in deities of one sort or another just teach one fewer "pretend" story to their kids. Jews and Christians teach them that "God" is a real thing, but teach them that Santa is a "pretend" person. Same with believers in other religions.
I just happen to feel the same way about deities. Children trust their parents, and will, generally, believe what their parents tell them over what they hear elsewhere. At least they will do this for a while. Eventually, however, if parents lie to children, they will figure it out, much to the detriment of the parents.
Religion, on the other hand, is something that can stick throughout one's life, and that's a good thing, in most cases. Most people believe in a deity or several deities, even as adults. That's fine with me, even though I don't.
Santa Claus, however, is not a deity. He is a "pretend" character, just like the witches, fairies, gremlins, and unicorns. Easy enough to teach that.
I wasn't arguing that Santa is real, and you didn't answer my question.
Surely you don't agree that it is the role of a substitute teacher to tell a class full of children that their parents are liars?
That's what happened here, a substitute teacher decided that she knew better what to tell the children. You teach your children what you want, I'll teach my children what I want, but the substitute teacher can just read the stupid poem.