Just did this.
Show some cool X-rays. At this age, probably just normals. Ask them if they can guess what body part.
Bring in some rubber gloves (blow up and make a balloon. "Cool! You made a chicken!")
Stethoscope: see if they can guess what it is, then let them hear their own heart.
Band aids (sounds dumb, but cool if the Barbie and/or Spider-Man kind, and you're 7, "awesome, stickers!")
Get a "What doctors do" book (10-12 pages kinda thing) on amazon and read it to them.
Bring in a 20 cc syringe ("this is for shots, but look, you can make a squirt gun out of it!" with water in it, obviously)
Bring a $15 home BP cuff from a drug store, show them how to check BP.
Bring in a 10 second digital thermometer (with the sheaths so no contamination)
Any spine, bone, or other cool model that you might have would be cool, also.
Most importantly, tell them about the scourges of throughput times, Press Ganey, student loans, circadian rythm shifts and medical malpractice lawyers and..Just kidding! Keep it fun, and innocent.
At the end, open it up for a few minutes of questions and smile as, instead of getting questions, you get 10 different versions of, "My brother had to go the ER to get a Lego taken out of his nose. Twice! It was cool!"
My kid still remembers this, and I found myself strangely less cynical and somewhat refreshed about profession afterwards. You'll be a minor hero for doing this. I'm glad I did it.