The God of Small Things finds our protagonists Estha and Rahel at two very different times of their lives: their childhood, in 1969, and their adulthood, in 1993. As a result, the tone of the novel contains aspects of the childlike and the mature. That isn't to say that the moments of childhood are characterized in a particularly light or fun way, or that adulthood is portrayed in entirely grown-up language. Just as the book itself skips back and forth, so does the tone. For example, the writing often takes on a rhyme-y, sing-songy tone, sounding almost like kids chanting while jumping rope.
Nevertheless, the moments in which we see this kind of language used are often moments in which serious information is being conveyed. For example, when we find out that the twins are 31, we also learn that Ammu died when she was 31. The narrator darkly rhymes that 31 is "Not Old. Not Young. But a viable die-able age" (1.18-20). Similarly, when we learn about Sophie Mol's "special child-sized coffin," the narrator describes it in a sing-songy manner again: "Satin lined. Brass handle shined" (1.25-26). The cuteness of such sounds and rhymes stands in sharp contrast to the tragic things they are describing. This mix of seriousness and lightheartedness is one way the narrator shows us how complex the events of the novel are for our characters, and how our young protagonists are forced to deal with tough issues at a tender age.