Not quite sure what to think about this one. Feel like I missed the changover that the 'shimmer' represented; maybe I need to read it again.
If it's a fantasy piece (which it essentially is), I feel there needs to be more work done to build the wonder of the world.
I found the dialogue jarring (too adult-like for kids of 10), the reasoning for the tanks being there (without a world of fantasuy built, I'm spending too much time trying tying to explain it), and the change to the moral lesson (represented by the 'shimmer') feels inorganic.
Would be keen to hear your thoughts on what you're going for here, as that may clear up some of my comments / questions.