A Kids level, is a level that you can solve at the first try.
(If it's really too easy, it belong to the Beginner level file)
 
An Easy level, is a level that you have to think a little bit before being able to solve it.
 
A Medium level, is a level that you have to try many times before  being able to solve it.
 
A hard level, is a level that you solve only after many, many, many tries.
(You play some easier levels and come back later to it)
 
A deadly level, is a level that look impossible to solve,  and you can try it for many, many days
before being able to solve it. (maybe never being able to)

Remember that even if a level has a very long solution this is not necessarily a difficult level,
and also, a short solution doesn't mean it's an easy level.
The rating should correspond more to the work you do with your brain
than with your keyboard and your mouse.