A stone circle and a barrow feature strongly in Alan Garners Elidor, the pinnacle of this kind of children's fiction in my opinion. Constantly surprised that it was never made into a film (and relieved after the mess they made of The Dark is Rising).
Recently read it to my 8 year old. It really is a scary book, was worrying that it might be a bit strong in this pasteurized present but it captivated him - scared him in all the right ways.