Using graphics packages
An exercise that works well with pupils, which allows them to try out the tools on any graphics package, is to draw an alien. As there is no 'right' answer, they can draw quite freely.
Using drawing programs is a good way to introduce young children to using the mouse, perhaps drawing a wavy line on screen and asking them to trace it with another colour. Here they have to move the mouse with a button held down. If the program has stamps then these can demonstrate how clicking once can cause something to happen. Using the selection tool can help them learn to 'drag and drop'.