Airasia — Son seated 16 rows behind

Hello, please see attached my boarding pass, your system automatically put my son 16 rows behind me by himself, he's only 3 years old. He ended up crying & sitting on my lap for the whole flight. Why did I have to pay for his seat? I asked 5 female agents who worked for air Asia from check in to boarding & they all ignored me & never got back to me. Ridiculous system & very bad experience.