Way back in the 80's, I saw this book in our local drug store and I was fascinated by the premise. I only saw the book once and then I could not find it again. The image on the book stayed in memory. I knew that I saw it and read the first page about Barbara the babysitter playing the piano. Then rms19 https://www.deviantart.com/rms19 used the image in one of his photo manipulations and had the original book cover and I so good that there was evidence that this book existed.

It was being reissued as part of The Paperbacks from Hell Series, a group of horror books from the 70's and 80's, and I bought a copy. It had the original cover that I remembered. The book is a bummer book, the ordinariness of the terror and horror is a little unnerving. It is a banal evil, where all of the actions follow a logical pattern that kids are testing the limits of their power. Barbara tries desperately to convince the kids to let her go before it descends into depravity and violence. It is unsettling because there is no supernatural element to blunt the horror, if there was a vampire, demon or evil spirit driving the action allows the reader to safely enjoy it. The reader is literally one of the kids that bound and gagged the babysitter. 

I do recommend this though it is a rough read. Here is a description of the book:

"They're just kids ... It's only a game." That's what Barbara, a lovely twenty-year-old babysitter told herself when she awoke bound and gagged. But the knots were tight and painful and the children would not let her go.

"They're just kids ... It's only a game," she told herself again. But the terror was real ... and deadly!

In the decades since its original publication, Mendal W. Johnson's bestseller Let's Go Play at the Adams' (1974) has gained a reputation as one of the most harrowing horror novels ever written, and copies have long been unobtainable except at exorbitant prices. This edition reproduces the original paperback cover art and features a new introduction by Grady Hendrix (Paperbacks from Hell), in which details about this cult masterpiece's enigmatic author are revealed for the first time.

Thanks to rms19 for helping me rediscover this book