Non-rigid airships (Wikipedia)
Blimps don't have a frame holding them together, as opposed to rigid airships. Blimps need a constant internal pressure that is higher than the surrounding air pressure to maintain their shape. The Goodyear Blimp has no frame.
Zeppelins are rigid airships, which means a skin is put over a frame. Because of the frame, they could be built larger and lift heavier loads. A Zeppelin (capital z) is a type of rigid airship (brand name, if you will) created by Ferdinand von Zeppelin.
Dirigibles are steerable airships (from the French
dirigeable, "steerable"). Wiki is sort of vague about the differences, and generally thinks dirigibles don't generally refer to blimps.
Hot air balloons use a different method, that of hot air is lighter than cold air. I'm sure there must be an easy science experiment somewhere on the internet.
How they work: Fill a bag with a gas that weighs less than the atmosphere. Add a fan in the back, some fins, and away you go.
The Hindenburg ("Oh the humanity!") was a Zeppelin. Unfortunately, due to a war shortage of helium, it was filled with flammable hydrogen. Normally they are pretty safe. Recently in the Boston area a blimp sank into the trees when it got a leak. Made all the news shows with helicopter shots of the blimp laying over the trees.
The way you can tell the difference is that blimps are LIMP. There is a story that first was the Type A-limp then the better version Type B-limp, which soon became the name "blimp". Wiki has a similar story, but also claims it is false. Since I heard it as true on a radio word-game show, I'll reserve judgement.
Don't think that I happen to know all this; credit Wiki for it. Except it's strangly quiet how Led Zeppelin got it's name....
I was curious what you ended up doing for the workshop. Just remember, (from my elementary school Math and Science Nights) you only have to know a
little more than the kids, and have plenty of trivial questions and answers. Cow magnets are my favorite.