My doc in general doesn't like to do bronchs "for the sake of doing bronchs." I agree with Tonya... yes, they are commonly done, but they come with a good deal of risks, risks that I wouldn't want to take for asymptomatic kids just for the sake of getting a specimen.
A throat culture is probably sufficent if they aren't coughing up much stuff anyway. If they can though, coughing up mucous and spitting it out is going to be more accurate in general. (you asked how, well that's how. it's pretty much as simple as cough + spit into the cup <img src="i/expressions/face-icon-small-smile.gif" border="0">)