Hi, thanks for your question. I am very glad to help with your querries. Non-visualization of the appendix excludes an appendicitis or a ruptured one, this is because a healthy appendix is usually non-visible on ultrasound scan. So you can put your mind at rest on that. However, the ultrasound finding of fluid in the abdomen coupled with the abdominal pain is suggestive of peritonitis, which is an inflammation of the thin lining covering the abdominal organs. This is usually caused by infection spread from an infected organ. You need to take your child to a primary care doctor as soon as you can for proper evaluation, treatment with antibiotics and pain relievers. I hope i have answered your question. Warm regards.