So I know the truth about Santa, but my parents say “You don’t believe, you don’t receive.” Which is a fun thing to do even if the older sibling (s) know the truth, but the younger sibling (s) don’t. I’d probably follow this trend until the talk happens with the whole Santa thing for the youngest kid, but I just say yes now you know the truth, but you don’t believe you won’t receive. Which then I’d do it until they graduate high school.