Hi Bo,
I've participated in many school kid star parties, from kindy to high school. Here are some suggestions you may like to consider:
* Look for an evening that has at most a first quarter Moon. This will allow for easier DSO observing.
* Have a few scopes available as stations, each on a particular target. Even if the folks who help out have next to no experience, the Moon is one of the easiest targets to re-aim a scope at.
* Choose just three or four bright objects. If some planets are visible, limit magnifcation to around 120X max as anything more and vibrations will make for poor image with novices.
* Have a short talk prepared to start with, pointing out some naked eye features of the sky along with whatever skylore that belongs to some constellations. If you can, a green laser pointer is a great tool. This talk will also serve you to lay the ground rules for how to treat the scopes, such as not to grab at the scope or eyepiece, and to note that some scopes may have their eyepiece located at different ends.
* Check if that night the ISS will be visible. It will cause a major stir!
* Don't use your best EPs! Grubby fingers, make up laiden eyelashes, and what ever type of microbes will be unleashed on them.
* Be prepared for very challenging coming from the kids. They will proove much more challenging and in-tune with all things science than their parents, even from kindy. The kids will be able to grasp very abstract ideas surprisingly easily.
* A video camera with a small monitor can be a good tool to view dimmer objects such as nebulae or galaxies. Novices will not be able to master averted vision to view these.
* Involve the teachers to have a themed night to create some excitment for the evening. Maybe ask the families to bring their own binoculars too as binos are very underestimated for astro. This will help keep folks entertained while they wait for their turn at a scope.
* Give them simple instructions on how to view into the scope while at the scope, such as "look into the glass window", and have them slowly bring their eye to the eyepiece. A little red light torch will help some people 'see' where they need to look into.
The Kindy session I had was themed as a "pizza and planets night"
. But careful crowd education saw everyone wash their hands before coming out to the scopes. My wife minded the school's 4.5" dobbie on the Moon with no prior scope experience and did a great job, my C5 tracking Jupiter and not once did I need to re-acquire it, and I focused my 17.5" on the Jewel Box. Some 100 students, parents, siblings and teachers had a blast.
You will be surprised how many people you'll be able to deal with with a few scopes and a couple of people to help out. The kindy session I was asked to host was my first "solo" session, and it was quite a buzz. The teachers that will attend will be your greatest allies, so your conservative number of 20 can be easily doubled.
If anything else comes to mind I'll post it too.
These sessions can be very rewarding for everyone. The sense of community they espouse is fantastic.
Mental.
If they are really young, they will need to learn to close one eye and look through the other. 
