i think you can take a look at Shoppers Stop. you can check in the women's section and go for the XXS, XS or S size. i am not sure about formals for kids, but Shoppers Stop has very good formals and some of the smaller sizes can easily fit a 10 year old.
my friend recently bought a similar attire for her daughter from an Allen Solly store. if you want to buy the pieces separately, you can also check online, but otherwise i would suggest take her to the store so that you can be sure about the fitting.
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