Book from 80s

[JK43830]

Hi. I am interested in finding the title of a book from the early 80s, maybe even late 70s. It was a book I read in third grade and was a RIF (Reading is Fundamental) book. It was about two children who were sent to live with their uncle or grandfather, I think for the summer and found a book and were told never to open it. So, they did anyway and nothing happened, or so they thought. The next day one of them went into the forest and never came back. When the other one went to look for them, they found out they were in another time. It was such a good book, or so I remember it to be and I was interested in knowing if anyone knows the title. Thanks!

 

[Ice]

It doesn't quite fit but my initial thought was The Magician's Nephew by CS Lewis. It involves two children, his uncle, a forest (the Wood between Worlds) and another time (the creation of Narnia).