Learn To Draw Cars is a free app that presents five-step instuctions on how to draw 10 different kinds of cars. It ONLY presents the instructions, though. The child doesn't actually draw on the device, so it's an app that has to be used side-by-side with a paper and pencil. That's not a terrible idea, but even in that respect the app doesn't offer much. Since there are only five per car, some of the steps are HUGE jumps that even an non-artistic adult like me would have some trouble following.
If your child can follow the steps presented, they're already a good enough artist not to need this app.