Monsoon Sleeveless Size 9 Corsage Belt Hi-Low Navy Blue Girls Dress
This from Monsoon's Party Perfect Children's Collection and now completely sold out. 
It would make a great blue themed flowergirl's dress or party dress.

From Monsoon UK Website:

This special gown is perfect for a wedding. Created in duchess twill, it features a scoop neckline, corsage belt and lightly dipped hem. Fully lined and fastens with a zip.

Our recycled polyester is made from PET plastic bottles which are formed into strings of yarn, before being dyed and woven together to create a soft-touch texture.

Introducing S.E.W: our commitment to seeking an eco-friendly world, starting with fashion. This capsule collection is created using sustainable fabric alternatives, including responsibly sourced organic cotton and LENZING™ ECOVERO™, a planet-friendly alternative to viscose.

I also have this exact same dress in Size 3