This book is about 2 children who traveled trough time using a circle of stones similar to Stonehenge. It was read to me in 5th grade, around 1985. I am having trouble remembering more details. They had to find a way back home at the end of the story.
There's a seven episode TV drama, Children of the Stones, that aired on the UK ITV network in Jan/Feb 1977 that has certain elements of your description. It was novelized (March 1977) in a book of the same name. The TV series is available on DVD (Amazon, Barnes & Noble, Netflix). A Kindle edition of the book is available but hardcopy appears to be out of print.
- Set in the village of Milbury which is in the middle of a megalithic stone circle.
- Has two teenage leads, characters are named Mathew and Sandra.
- Involves temporal paradoxes and something called 'time loops'
All the references say it was definitely frightening. Does that aspect ring a bell ?
Could be the Wierdstone of Brisingamen? Not sure if the time travel part matches, but there are two kids involved. Susan and Colin are siblings, staying with a friend of their mother while their parents are overseas. Susan's bracelet contains a mysterious stone, the wierdstone of the title, unbeknownts to her until an evil spirit tries to steal it from her. Wizards and prophecies, and an ancient evil, elves, dwarves, and ancient gods, with mixes from norse and celtic mythologies make for an interesting story!