For Ma this journey is fairly clear, but her life before what we see in the book must be taken into account. The "ordinary world" at the beginning of her is the normal world outside, with no Old Nick or Room. Then, she is kidnapped and everything changes. In a way, this kidnapping functions as a "call to adventure" in that it starts the heroic ordeal of living in Room. Also, Ma's attempts to escape from Old Nick can be seen as a classic "refusal of the call." There has been no shortage of heroic challenges so far in Ma's "journey," and we have yet to read how this journey concludes.
Jack undergoes a very different kind of hero's journey. He is born into Room and it is all he knows, so the world view he has at the beginning of this novel can be seen as his starting point, or "ordinary world." His heroic task is then to break free of this mindset and to break free of Room itself. In Jack's case, I think the call to adventure comes from Ma starting the process of "unlying" and starting to make Jack understand just how small his world is. When Ma first tries to explain this to Jack, he resists her explanation, refusing to acknowledge that there is more to the world than his mother and him. However, at this point in the novel, he is on his way towards escaping his narrow worldview, and perhaps even Room.
Perhaps these characters do not fit the paradigm of the hero's journey perfectly yet, but there is still a lot of development left in the novel. Do you think that Room would be correctly called a hero's journey narrative? Which of these characters is closest to the idea of a campbellian hero?