What makes a high fantasy realm appear real? Myth, legend, traditions and history. These may not be the whole story but they make a huge difference. Take Earthsea and Middle Earth; I've never come across a complaint that these places don't seem real, despite magic, dragons and kings who come from obscurity into their inheritances. Both have songs, poems, languages and history that merges into myth and legend...just like our world.
The world visited by the titular child in this fantasy does not have these things - it appears really thin, as if the reader would just find empty, grey mist if he or she could look "off-stage". The blanks have not been filled in. There's nothing there outside the action. This is one problem with this book. There are others.
The Green Boy is a prophesied hero, but there is no explanation of why a hero is needed at all; the events at the climax of the novel do not seem to be dependent on his presence; any other person, or nobody, would seem to work just as well.
At one point a pair of "bad guys" are killed more or less because of the actions of the protagonists. Later many more are killed. The characters feel nothing about this. Nothing at all. No remorse, guilt or even angry joy. They have no reaction at the time or any later time. How can that be? People usually have a very strong reaction to killing some-one, even in self-defense, even when the responsibility is indirect. It's just unbelievable that these kids carry on as if nothing of the sort happened.
The otherworld, Pangaia, is apparently an environmental disaster. This the fault of scientists and the government. I'd like to take every person who believes scientists are responsible for this planet's ecological woes and ban them from using anything invented after the time of Francis Bacon. Maybe even Roger Bacon. Or Archimedes. Scientists discovered the cause of acid rain, the cause of the ozone hole, that humans are altering the climate of the entire planet. Having discovered the problem, it is up to all of us to solve it. It is easy to blame the government, but at least in some parts of the world, we choose our own governments, and they are a reflection of us en mass, however distorted an image they may be. We are all responsible.
One of the "scientific" woes inflicted on Pangaia is genetically modified plants and animals. This, like so many other problems supposedly caused by scientists, in reality is a problem of engineering purely for profit. So maybe we should blame it on corporations. Government should properly regulate them. Oh, yeah - that means we, the people, should ensure proper regulation by choosing our representatives carefully. People who understand the difference between science and engineering. People who are capable of understanding the scientific process and the results of science. For that we need people who are properly educated - people who don't look on education as merely a method of reducing unemployment figures. People who recognise the equation connecting resource use, population and environmental impact - not just to be our representatives, but to be able to choose our representatives. No-one is going to give us that education - we need to go get it ourselves. It's hard work. Much harder than blaming "them", whether "them" is the government, scientists or corporations. Is it harder than writing simplistic novels with misleading and irresponsible messages, novels that don't provide any practicable solutions?
Cooper does her usual thing, mixing up various myths and notions from our world to concoct her fantasy realm, the Green Man, Gaia, but it's all wasted here. Read The Dark is Rising, instead.