Either get your child to draw some picture outlines in crayons or you can provide the templates.
Leave the paint in the ice cube tray and allow it to dry out. When you want to paint again just add some more water and the paints will be ready for round two (even if you aren't)!
This means that we have a constant supply of pens that I seem to be throwing away on a regular basis. That is until I came across this idea.
By putting similar colours together, e.g reds and oranges, or blues and purples, we had enough to make home made water colour paints. Simply immerse the pens in about a quarter of jar of water and leave them for about one week. You will see the colour of the water gradually changing as the colour from the pens leaches out.
Tip a small amount of the coloured water into ice cube trays and use a few drops more water to dilute it if the colour is very strong.