I think that's a good idea. I don't think being at one school from 5-18 is the best, and you if you want to change, you may find you're not as well prepared for other schools' exams (makes sense I suppose, they don't want you all to leave!)
Ok, some of these are to 11, some to 13, and some are mixed and some all girls.
Wilmslow Prep I went here, and loved it. Obviously wont be quite the same now, but it's a lovely school location and buildings-wise and everyone I've spoken to seems happy there.
Pownall Hall Also in Wilmslow, quite good record I think, lots of speech and music.
Ryleys In Alderley. Has recently started taking girls. Still runs to 13, so good if you want to board afterwards. Not so sure how academic it is (it wasn't great last I heard), but no girls there are old enough to be sitting for senior schools like WGS/MHSG yet so I don't know about that.
Ladybarn Academic (if a bit pushy imo), but much larger than the others. Not so good if your DD's shy, my brother moved here from the Ryleys, and I always thought I'd have hated it.
There's also Terra Nova, but it's a bit further out (Holmes Chapel) and probably better if you want to board.
Or there's Bowden and Hale, but again, a bit too far I would think.
Hope this is helpful.