We had similar problems with DD1 a couple of years ago, she was quite tall & long in the waist. I resorted to urban angel (the tween part of pumpkin patch) for a while as they seemed to have a bit a stretch & elastication which improved the fit, and a range of lengths.

The other thing you can do is get layering - leggings etc under shorts & skirts. Same goes for tops, which seem to get ridiculously short in the tween clothes ranges. Having some longer tanks etc to wear underneath can make all the difference.