The first example of an animated film which still exists was an advert for matches; made around 1893. It used the pixilation technique of shooting real objects single-frame in 3D space as against drawings.
Until the advent of animation-cel (around 1915 I think), Pixilation was the main form of animation and primarily for Special Effects as in the 'Keystone Cops' films.
Wikipedia describes it as:
Pixilation (from pixilated) is a stop motion technique where live actors are used as a frame-by-frame subject in an animated film, by repeatedly posing while one or more frame is taken and changing pose slightly before the next frame or frames. The actor becomes a kind of living stop motion puppet. This technique is often used as a way to blend live actors with animated ones in a film.
It is the simplest form of animation, and can easily be done using a digital camera with a single-frame button. But can also be done with an analogue movie camera just by clicking it on and off.
With children, the most common idea is to let them make a funny face for each shot.
This type of animation requires no editing and offers instant playback, but can easily be extending to give 'magical' effects such as people and objects appearing and disappearing, or moving around at a frantic pace.
This technique can be automated as 'Stop Frame' shooting, where the camera takes a shot at fixed intervals. Commonly seen in CCTV cameras, and scientific films where you see a flower growing in a few seconds though it actually took hours or days.
It is an ideal starting point for animation in schools, as once set up it requires no further attention.
A good site for learning basic techniques, including Pixilation is http://www.anim8ed.org.uk/resources_tech_pixil.asp
This site also gives instructions on setting up school projects using a variety of techniques.
More information on: Pixilation