We never thought Santa was real (yes I know about the Bishop of Myra) as children and would not teach it to our cubs but there is no reason to act like this.
In fact as children we were taught that some children believe in Santa and to be kind we would not correct them. Which is the same thing we would teach the cubs.
If you want to argue with the parents do so, but leave the small fry out of the conversation.
He's making a list,
Checking it twice,
Gonna find out who is naughty or nice,
Santa Claus is coming to town.