I think leggings with pockets might be a dream (since leggings are tight fitting).
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That said, if she is up for dresses, Land's End has girls dresses with side pockets my DD always liked (and my DS for that matter, lol, he had one he *loved* that was bought just for him).
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This one is on sale for $15:
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http://www.landsend.com/pp/JerseyDress~223936_1186.html?bcc=y&action=order_more&sku_0=::ILS&CM_MERCH=IDX_Girls-_-DressesSkirtsSkorts-_-LittleKid&origin=index
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http://www.landsend.com/pp/JerseyDress~223936_1187.html?bcc=y&action=order_more&sku_0=::ILS&CM_MERCH=IDX_Girls-_-DressesSkirtsSkorts&origin=index
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We were also lucky with dresses/skirts at Hanna Anderson and pockets.
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For lightweight pants with pockets, you might look at capris made with lighter material (light cotton, seersucker...). If you have the $ and inclination, REI has very lightweight pants with TONS of pockets (since they're made for hiking) that zip off.
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Good luck!
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