Children with phenylketonuria cannot eat fish and shrimp because phenylketonuria is caused by a deficiency of phenylalanine hydroxylase in the phenylalanine metabolic pathway, leading to metabolic disorder of phenylalanine in the liver. Phenylketonuria is the first hereditary metabolic disease that can be controlled by diet, as natural foods contain certain amounts of phenylalanine. Therefore, once diagnosed, children with phenylketonuria should stop consuming a natural diet and should be treated with a low phenylalanine diet. Fish and shrimp contain rich protein and high levels of phenylalanine, hence children with phenylketonuria cannot eat fish and shrimp.