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.