I've been having the same thoughts about getting a bow for my 8 year old daughter. I am considering the Freestyle from Fuse archery. It is adjustable from 10-25lbs and from 14-24" of draw without a press. It is available in a kit with everything she needs for about $150.00 online. I think it would last long enough for her to decide if she's serious about shooting a bow. If she likes it I would move her up to a Mission (Mathews) Menace, which is similarily adjustabe, but can reach hunting type draw weights.
Did I mention that the Freestyle comes in pink camo. I don't suggest showing it to her unless you want to buy it, a mistake I've already made.
http://www.fusearchery.com/youth/index.php