You have a really fundamental flaw here, which is trying to design a game for age 5-18. It cannot be done, the design considerations for a 5 year old and an 18 year old have almost nothing in common, you need a far more focussed age range. Like 5-8.
So before thinking about how to indicate difficultly, I'd really urge you to split your demographic and make separate games, or even just scrap two thirds of the age group that you are trying to hit.
Next, if a child perceives something as being slightly too young for them, they will regard it as babyish and will not be interested at all. So whatever you do, do not use the above suggestion of big blocks of bright primary colours. Give that to a 14 year old and you'll be laughed out of the building.
The general rule when designing for children is to age up the visuals and age down the interaction. So when thinking about the information you're trying to communicate, make it as simple and literal as possible, but then when creating the artwork, keep a much higher age-group in mind.
So for indication of difficulty, bearing in mind the above - if you are dealing with young children, you cannot rely on them reading anything, and you cannot rely on them understanding ANY metaphors. You must be as simple and as literal as possible. Forget about trying to find a metaphor to represent difficulty, and go directly to the source - create something that is a direct representation of what they will experience when they hit that button.
If a higher difficultly means enemies moving more quickly, show enemies moving more quickly on the button (eg. motion blur). If it means only 3 lives instead of 5, show three lives on the button.
If you are designing for children there are two excellent resources, the recommendations in both of them perfectly match everything that I've ever seen when testing with kids -
http://www.sesameworkshop.org/assets/1191/src/Best%20Practices%20Document%2011-26-12.pdf (may be too young for your audience)
http://www.nngroup.com/reports/children-on-the-web/ (costs money but highly worth it, there's a very short free summary here: http://www.nngroup.com/articles/childrens-websites-usability-issues/ and also a version covering teenagers here: http://www.nngroup.com/articles/usability-of-websites-for-teenagers/