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If she is crawling, then I would say avoid a dress. They just aren't practical.

Ditto to what @T.H.O.U.: said about dresses. They used to just trip my daughter up when she was crawling. I'm assuming the party is soon so I would go with a cute tunic and capri leggings or cropped trousers and a matching sun hat. I will try to find some pics. If she's not going to be crawling around a lot, then you could go for shorts or a romper. I'm just all about keeping some kind of cloth over their knees when their in the crawling stages.

I agree with the no dress thing, my LO wore one to her first birthday and she was still crawling. I had to tie it with a hair tie and it didn't look pretty. Maybe a cute romper?

Gap has some cute ones- some on sale. http://www.gap.com/browse/category.do?cid=7190&departmentRedirect=true#department=165

And there is always etsy.
http://www.etsy.com/listing/154608695/salechevron-romper-headband-baby-rompers?ref=sr_gallery_4&sref=sr_964a06658539baafc0ca3da865e232e3e1c60424f25db4c696d96f8cc69db474_1373911395_14440458_baby&ga_search_query=baby+girl+romper&ga_order=most_relevant&ga_view_type=gallery&ga_ship_to=US&ga_ref=auto1&ga_search_type=handmade
http://www.etsy.com/listing/126050019/baby-girl-romper-hot-pink-floral?ref=sr_gallery_19&ga_search_query=baby+girl+romper&ga_order=most_relevant&ga_view_type=gallery&ga_ship_to=US&ga_page=2&ga_search_type=handmade