my daughter is eigh too. made the bow as long as her.
dont make the bow too strong, they want to ( and you want em to as well!!!) shoot for ages, dont make it too powerful that they tire of it , and quit it.
made my daughters 10 lbs. she can stump shoot heaps with me. first used the red glass wally mart arrows- horrific things, and the worst thing i could have done for her. they shot badly, and wouldnt strick well in a butt, from such a low power bow. so bought some 5/16 dowels, got some 100gr field points and judos, she painted up her own arrows, she loves em , and shoots really well now.
good luck
and best wishes for shootin with the wee lass.