Today I used a story book recommedned in the 'Roots' material for our 'all together' bit of the service:

This little book, seemingly out of print, but available second hand, offers a child-friendly approach to thinking about the mystery of God.  Not trinitarian but not anti-trinitrian either.  Beautifully written and colourfully illustrated... so long as you can deal with English (USA) it is a delight.