Most of the main firms do a range of back lengths. My daughter was fairly tiny at 7 (still is at 17!) and she had a Aerowear revier extra small at that point. Looking at their size chart the smallest possible is Child XXS for a 54cm chest size; shortest back length is 44cm. That reads to me a very tiny! Their size chart is here http://www.airowear.co.uk/en/product/reiver-010/