Q:

older scary story about drawing things that appear

It must have been very early 70's or even late 60's. I read a scary children's short story about a boy and a girl who make drawings that manifest themselves (overnight?) In the end they mess up and the house they live in is surrounded and threatened by hostile rocks/boulders and I think they have no escape. I think the boy drew the rocks with angry faces. Sounds silly but it was quite frightening and may have been one of the first stories I read where the "good guys" lose, or I stopped reading too soon.

A:

"Marianne Dreams" by Catherine Storr, published in 1958 ticks a lot of these boxes. The protagonist, Marianne, falls ill and has to spend months convalescing in bed. To pass the time she draws with a pencil she found in her great-grandmother's work box. In particular she draws a house with a boy at the window, and finds that in her dreams she can visit it. It turns out that the boy corresponds to a real boy called Mark, suffering from polio.
As you recalled, the house is surrounded by boulders. She cannot erase anything she draws - she can only add to the picture - and one time she drew angry eyes on the stones, turning them into "Stone Watchers" and giving them malevolent properties. These stones prevent the children from escaping.
It actually does have a happy ending, although the book has a very dark tone, and at times things seem hopeless. 

 Marianne draws a lighthouse (a beacon of hope) and they manage to
 break though the cordon of Stone Watchers on bicycles.

