The fabric looks like jersey knit and bodice is simple, but not plain. Love the sweetheart neckline and off the shoulder sleeves. The skirt is a bit poofy and was perfect for the scene in the park - Visit to grab an amazing super hero shirt now on sale!
It has been noted that the Himalayan blue poppy is the national flower of Bhutan, so obviously the LK Bennett 'Lasa' poppy print dress springs to mind. However, I doubt Catherine would make such a recognizable repeat.