vinnie90 - I find that normal-ish parts take about 4 hours...that is something around 5 inches in size.

If you have a printer that you could borrow and show kids with free google sketch up to make the 3D model.

toybuilder - If the idea is to give them a combined intro to 3D modeling and printing, and the audience is relatively inexperienced, sticking to basic geometry in TinkerCAD is a great way to do this. There's no software to install for the 3D modeling side, and they offer (or used to, anyways - haven't checked recently) team accounts that make it easier to manage the files within your group.

Simple designs with low infill can print fairly quickly, especially when printed with thick layers. For 0.4 nozzles, for example, I'd bump it up to 0.32 height and possibly experiment to 0.35 height.

The only problem is overhanging features can be difficult with this -- but that's probably an okay thing for the class to make the students understand that they need to look out for overhangs.

Since kids lose focus and interest fairly often, break up the class with different segments with checkpoint breaks in between. Use the first third for guided instructions. Time your segments so that you will have one or two simple and smallish designs that you can print during the lunch break. Then get into more advanced modeling and pick one a design that you can kick off at the afternoon snack break (but might continue to print for the rest of the afternoon). Use the rest of the time for more self-directed designs and guide them on potential problem spots in their designs. Collect completed designs and print the renders on paper to take home, and hire out the printing of the designs to get them all made in the next few days so that they have a finished product in their hands.

...at least, that's what I'd do! :)

tanju-b - I would print small trinkets from thingiverse. I print minecraft creeper head keychain scaled to 80% as demos all the time. It prints in about 10 mins and the kids love them.
I recommend tinkercad.com for introducing kids to 3D design.

simon07 - I would say students have a lot of time period to design a 3D model within a few hours. Now students can also take online designing font help with the https://masterbundles.com/fonts/serif/handwriting/ option where handmade written fonts are available for the best quality designs and graphics.