I made Sienna's flower girl dress in 2 hours back in March and let it sit while I did all the other sewing.
The pattern is Simplicity 5695, which I have made several times (notably the Lucy Dress and the dress Maggie wore her 3rd Christmas). It's quick and easy. She likes it!
The dress needed some pouf. Maggie is wearing a poufy slip in the Christmas dress pictures, but I don't know what happened to it. So I freehanded a slip and then added a folded, gathered strip of petticoat net around the waist. And a snap at the back of the neckline to hold it up, haha!
The wide sash I tried originally looked terrible.
So I cut it off and we went with a strip like a ribbon.
The hem is a rolled hem, because I think rolled hems are awesome.
The sewn on sash didn't help make the dress cinch around her waist, so I ended up sewing little seams at each side to make it smaller. And a bow to match Maggie's! It's the same size, using Simplicity 4647.
See also Maggie's Flower Girl Dress (Simplicity 4647) and Dalton's Flower Guy Vest and Bow Tie (Simplicity 1509)