Vehicles Coloring Book is a nice coloring application for kids which includes lots of 3D vehicles for them to paint. You will see all types of cars, cranes, trucks, buses, etc. To paint, the kids just need to click on the desired color and then click again on the part of the drawing they wish to paint. Sadly, the program doesn't include too many colors, but kids can create more by combining the colors on the palette they'll find in the bottom-right corner. If you wish to clean the brush, you can use the water jar. Note that the trial version will only allow you to plaint the first two drawings.
If you make a mistake when painting you can undo your last move or clear the whole drawing and start again. Unfortunately, the drawings cannot be saved or printed, which is really a shame. The application is run in full screen, so you kids won't have access to your computer, and it offers nice graphics and sounds, and pleasant music.
All in all, if you are looking for a nice coloring application to keep your kids busy for a while, Vehicles Coloring Book may work for you.