Either would be fine depending on how they are visually represented. For example if you do apprentice/expert having a character pop up that looks a little more epic at each stage would be great for young people and easily understood. With the stones/gold, silver etc. some sort of means of measuring the value of each stage would be great. At the risk of sounding a little childish I'm going to reference Pokemon black. Each time you earn a badge its shown in your badge case alongside any other badges that you've won as well as empty space, or a dark shape where you still need to earn badges. If there are only a small amount of stages I would suggest the apprentice/expert idea. If there are going to be many I suggest the ruby/gold reward system. Both are suitable from my interpretation of the project, it's the manner in which they are awarded and display the progress that makes it simple to understand.
Hope this helps :)