Final answer:
The incorrect instruction is getting cervical x-rays between 3 and 5 years of age, as this is not a standard part of health maintenance for a child with Down syndrome. The focus should be on heart health, dietary needs, and monitoring for respiratory infections.
Step-by-step explanation:
The instruction that is not correct for the parents of a 1-month-old girl with Down syndrome is Getting cervical x-rays between 3 and 5 years of age. This recommendation is not standard for the health maintenance of children with Down syndrome. Instead, standard care includes getting an echocardiogram before 3 months of age due to the high prevalence of congenital heart disease in these children, adhering to any special dietary needs that may arise due to metabolic issues or growth concerns, and monitoring closely for symptoms of respiratory infections as individuals with Down syndrome are more susceptible to such illnesses due to their immune system differences and potential anatomical differences in the airway.