The best way of providing feedback such as this is through TEXT and therefore the image can be whatever you would like.
To use an image means that you must provide something that is universally accepted as a method of determining difficulty and understandable to children, this is a very hard thing to source.
Images are also completely open to interpretation by the individual. For example, where a frown may mean difficulty to one person, it may mean anger to another.
Text allows you to easily explain the difference in levels so it is universally accepted. The image can then relate to the design of your game so it fits with the rest of the user experience and interface.
You should use text such as EASY, MEDIUM, HARD, ADVANCED so that it ties in with a learning based program or if they are actually Levels that you need to complete in order, you should use LEVELS.