Not sure about this, but wondered if it could have been William Croft Dickinson's Borrobil? It was published as a Puffin in the 1960s and that was the copy I had but according to Wiki was first published 1944. It's set in Scotland, I think, and the brother and sister dance around the Beltane fire and then Borrobil who is their guide, and indeed rather Puck-ish, and they end up in a fairy hill at some point. I have not re-read it for years, though, and it's the opening I recall more than anything else. I still have a copy.