Here is a an Ariel costume from The Little Mermaid. If you’re looking for a sexy costume this is a nice choice. With Ariel you have the orange wig, the seashell bikini top, and mermaid bottom. Optional starfish in hair.
Here is a sexy Chun-Li costume (cosplay by enthusiast Meg Turney). The great thing about this cosplay is the detail of the victory celebration. The hand to the mouth and the weird jumping form make it perfect.
Here is a classic Disney princess- Cinderella. The dress is beautiful. See how the dress comes to a point in the front near the bottom of her stomach? That is a great way to look skinny…though this girl doesn’t look like she needs help.
She has a cute Heidi/ milkmaid braid and a servant to hold up the light blue gown. She looks rather elegant. I can’t find this exact gown (the style or cut is called “A-line”) but for the costume try here or for your kid-here. Or just buy a prom dress here, here and here. It is probably the same price.