Guardian: ‘A four-year-old child could understand this! Run out and find me a four-year-old child.” For the past year or so, this Groucho Marx line from the 1933 classic Duck Soup might well have been tattooed on the inside of my eyelids. This is what happens when you undertake the challenge of writing a book for children about a subject as complex and, at times, depressing as climate change. With every turn of phrase, I’ve had to remind myself that we must never underestimate a child’s intelligence, …