Belle was still asleep when she went to dream world, her subconscious traveled. But Emma and Regina–their very bodies and beings–traveled to this AU.
Not every time, unless she was able to sleep during labor, but I guess her subconscious did travel so maybe Belle was daydreaming? I don’t know, a dream world is the only way to make it make sense.
I’ll just leave this here, then. Make of it what you will.
HOROWITZ: It’s the wish realm. It’s the realm that was created by the wish, so it’s an actual place that exists now. The wish has created it, so it’s not an alternate realm in the sense that it’s just imaginary and not real; this place actually now exists and is real.
http://www.ew.com/article/2016/12/04/once-upon-time-robin-hood-gideon-spoilers