when and how to measured blood pressure in children

Begin routine blood pressure (BP) measurement at 3 years of age.
Correct cuff size depends on arm size. Practically speaking, correct cuff size equals largest cuff that will fit on the upper arm with room below for the stethoscope head.

BP measurement by auscultation is the Gold Standard.
BP by automated device correlates reasonably well with auscultation, with practical advantages of rapid measurement remote

BP by automated device correlates reasonably well with auscultation, with practical advantages of rapid measurement remote from child and elimination of reader error.
If BP is high by automated device, repeat by auscultation.