I regret to say I don’t remember doctors or nurses doing this on me when I was a kid…still, I’d like to know how they go about this. Obviously psychology is involved. What do you tell a kid so as not to scare him about putting that strange rubber thing around his arm, and squeezing it? The kid won’t understand the word “sphygmomanometer,” and won’t like the idea that someone is going to “take” something from him. How do they work this?
How young are you talking about? Do they recall ever being to the doctor? Don’t tell them you’re taking their blood pressure. Tell them the doctor is measuring how strong their heart is. Just like measuring their temperature (I assume they know about that or will by the time this happens), but they have to do it differently. Tell them the cuff will squeeze their arm just a little bit, but their heart is going to squeeze back so you can see how strong it is.
Okay…when my doctor’s assistants get my BP reading, I see several sizes of BP cuffs in the little basket attached to the machine; one is in a colorful print pattern obviously for little kids. So I wondered how they deal with them…You’ve answered my question. Thanks. 
We also don’t routinely take children’s b/p unless there is a reason to. An illness or trauma can affect b/p but something like an ear infection isn’t likely to have much effect.
My rule of thumb in the ER is if they’re old enough to understand ‘this is an arm hugger, it’s going to tell me how big your muscles are, but it won’t hurt, it’s just a hug’, they usually get a B/P. If they’re too young, but are lethargic or unresponsive, they get a B/P. If they’re too energetic and uncooperative they probably don’t need a one since HYPERtension is quite rare in kids, absent a congenital heart defect or a stimulant ingestion it’s not worth the effort.
And, yeah, they make cuffs that would work on my pinky.
My four-year-old has watched me get my blood pressure tested in the supermarket. He would love it if that machine were adaptable to his little arm.