It's fiction. Kensuke's Kingdom takes you into a fictional world crafted by the author's creativity. There's no real-life basis for the story or its elements.
Their relationship could be a bit rocky at first. They might argue about how to raise the baby. Asuka might be more strict while Shinji is more lenient. But through these disagreements, they gradually learn to compromise and in the process, their relationship deepens. They realize that they need to be united for the sake of their child.
Indeed, 'Kensuke's Kingdom' is based on a real story. The author uses his personal experiences to create a vivid and believable world. He might have met someone like Kensuke in his travels, or perhaps experienced the feeling of being in a strange and unknown place himself. This real - life basis helps the readers to connect with the story on a deeper level, as they can imagine that such things could really occur.