Er...yes. That is the movie I need help identifying.
When I was a kid in the 70s I inadvertently watched this really creepy flick. I was so young I barely remember it. I just seem to recall:
~ a family of four driving through the woods. They seemed nicely attired, though not too fancy. Sunday best, maybe. The styles and the mom's hair put me in mind of the 60s/early 70s.
~ a stony building (castle? convent? monastary?) which I think was also in these woods.
~ maybe the family's car broke down or they just stopped for a picnic, but the two kids (boy & girl, perhaps) went off exploring, stupidly
~ the kids arrive at aforementioned stony place and sort of peek through what I think were cellar windows
~ only it's not a dark, dank cellar, but a huge white room, like, ballroom size. There are two rows of women attired as nuns, wimples and all (or maybe flying nun type hats?). In the center's a massive black cauldron/pit. At the head is...possibly a priest?
~ not sure how or why but suddenly there's a roaring huge fire in the cauldron/pit, the nuns are running about chaotically, sort of jumping into and out of the fire, and their faces are bleeding.
It's quite possible I'm remembering this all wrong or have synthesized elements from multiple movies into one memory. Whatever the case, this thing traumatized me then and still haunts me today in my 40s. I feel like if I could watch it now I could exorcise the dread in my gut when I think of it.