I have very poor recollection of it, but it bugs the hell out of me!
Boy and a girl. Herefordshire or Welsh borders. Go out looking for mushrooms in the early morning and find patterns on the frost leading to….. something supernatural. In order to prevent discovery by persons un-remembered (presumably evil) they go out with hot towels and melt the frost to hide the pattern. The girl may have been called Jane, and there may have been dragons, but honestly any of it could be suspect after all this time, so hit me with anything close. I probably read it late eighties, so it’s likely a late seventies/early eighties release, as I only read library books.
Why it made such an impression when I remember so little if it, I don’t know. It pops into my head every couple of months, despite the fact that I can’t remember where I put my bank card on the regular! (It’s driving me crackers in a repetitive, low key sort of way, and I’m overwhelmed and having a shit time in real life, so escaping onto kids books is about as good as it gets just now!).