I'm trying to find a YA novel that I read sometime between 1977 and 1980, from our school library (after I discovered SF&F in second grade, but before I left that school). Two notes:
I was at a smaller school with a lot of older books, so I strongly suspect that this novel may have been written any time after the mid-50's.
Although I was definitely under age 10 when I read it, it was more-than-likely targeted at the YA crowd, since that's where I gravitated to in the library at the time (much to the frustration of the librarians who tried to guide me back to more "age-appropriate" reading material).
Points that I remember include:
Setting was contemporary to when the book was written; that is, twentieth-century setting, not medieval or futuristic, etc.
Protagonists were three physically-disabled children in a hospital (as I recall, they were unable to walk and confined to bed and/or wheelchairs). From what I remember of their description at this point, I suspect they had polio (also contributes to the possible 1950's vintage).
I believe that the children were two boys and a girl, possibly orphans (which means the hospital may have been an orphanage).
One of the children learns to "travel" to another world and teaches this skill to the other two children; initially, it's unclear whether this is a real world or an imaginary place.
As they go back and forth between worlds, they regain their mobility.
As I recall, the other world had a pinkish hue to the sky.
Once they finally see a glimpse of the night sky, they realize that they haven't learned to travel between worlds, but between times, as they recognize the moon, but it appears larger.
At one point, the boy who initially learned to travel forward in time returns to their home time one last time, but falls to the floor and hits his head hard, due to the fact that his bed is no longer there, since the hospital/orphanage has been closed and everything removed; this at least temporarily prevents him from travelling forward again.