Why are the kids shown the dark Thing on Uriel first?

From the top of the mountain, the children get the first real glimpse of the evil they will try to combat. This gives them both perspective and distance to contemplate evil. It is worth noting that the children could not see the evil darkness until they flew to the top of the world, out of the normal atmosphere. L’Engle suggests here that humanity is actually blinded to the reality of evil in the world - not just microcosmic sin, but the macro-form of sin and evil inherent in mankind. The stimulants of the everyday world has distracted us from the sin we commit in living our lives so blindly.