My six year old daughter always has her blood pressure checked since she was a little bitty thing.  Wether its the endo, pediatrician or the urologist, they all check it and let each other know if anything is going on, no matter how little it is. When she was little we used to have a hard time getting her to be still, we also bought a little doctors kit that came with a blood pressure cuff, it was the best thing we ever did. Never had a problem after that. My daughter also loves going to the store and having her blood pressure checked on one of the machines by the pharmacy. She knows how to sit still and turn it on and everything. She has gotten to the point where she rocognizes the numbers if they are high or normal. 

Tina