This was a Scholastic-type book, probably about 5th-grade level. I would have read it in the 1980s.
I remember there were a brother and sister, possibly in upstate New York. They either found or fell into a cave, inside of which were hazelnuts. They ate? the nuts, and this transported them back to Colonial times. They ended up living with a friendly family until they could get back to the cave/hazelnuts to return to the present. I think the settlers were Dutch or German.
There was one bit I remember vividly: upon arrival, they were taken in by a family, and saw something like a coal scuttle. When one of them asked what it was, the neighbor laughed and said something to the effect of "Well, if you're living with this family, no wonder! A coal scuttle is for carrying live coals from one house to another when your fire goes out, and Mrs. [Name] has never let that happen! Her fire has burned for years!" Later in the book, the family has rushed out for some emergency and forgotten to bank the fire. They run back, the woman feeds and fusses over the fire, and finally finds one coal poking out like a glowing eye:
She sat back on her heels. "X years," she said proudly, "and still it burns!"
There were a few monochrome illustrations — maybe in brown? — and the one I remember is just after the two have transported back to the present. The brother is standing and the sister is sitting on the ground of the cave, knees up, head on her crossed arms, and he notices her Colonial-style dress and her long hair spilling over her shoulders and back.
I would really like to read this again, so please let me know if this sounds the least familiar. I've Googled every combination of keywords I can think of.