According to Health Central, the normal systolic blood pressure for boys and girls between 8 and 12 years of age can range from 110 to 120, while the diastolic pressure may go from 70 to 80. Blood pressure in children varies depending on such characteristics as gender, age and height. On average, girls and younger children have slightly lower blood pressure than boys and older children.
Only a medical professional familiar with a child's history can determine whether his or her blood pressure is normal. Doctors diagnose high blood pressure in children only after blood pressure measurements have detected high numbers on three separate occasions.