Check out a game-like environment with images for colouring. View the instructions for children of different ages to select colours and fill in the areas for colour marking, adjust the results to get the optimal quality of the picture, switch between them at will, etc.
Color by Numbers - Animals is a lovely coloring application for kids that includes lots of drawings with animals to paint. The application offers three coloring methods: Paint by numbers (in which you color an image following the numbers), Color it to make visible (you also follow the numbers, but the image is divided in multiple small pieces, so you need to color it to discover what is in it), and Color it in your favorite colors (you can paint the image the way you want). The palette of colors includes only a few colors, but you can customize it from the upper bar.
The program allows you to undo the last change or clear the whole picture to start again. Unfortunately, the trial only allows you to paint one image, but there are lots of them in the registered version. A good thing is that the program is run in full screen, so the kids won't have access to your computer. The program features a lovely user interface with simple sounds and nice music.
In short, if you are looking for a nice coloring application to keep your kids occupied for a while, Color by Numbers - Animals may be a nice choice.