I remember a library book about a group of children who discovered they had magic powers. I don’t believe it was any of Edward Eager’s books, and I also don’t think it was Mrs. Coverlet’s Magicians.
The tell-tale sign of magical ability was being able to kiss one’s own elbow. (Spoiler: I can’t.)
Can you help? I would have read it in the early 1970s, but I have no idea when it was published. I’m guessing the 1950s.