They need to consider how much energy is required to get there - will they need a spaceship and what are the effects of gravity?
Think of their own questions about going to the moon.
What shape is the moon and how big is it?
They need to think in terms of three dimensional position and these questions stimulate this spatial awareness. It would help if they have an understanding of measurements - particularly length. They should also know about shapes - particularly 3D shapes.
Of course they may want to go somewhere closer at hand - try to make that involve travelling in three dimensions.
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