I think you might struggle a bit with this as this a traditional bride’s colour. Have you tried Debenhams, and Monsoon? It may be that you will need to buy actual wedding dresses if you are set on this colour.
Could you find someone who would make them for you? Are they very different in size, as some 12 year olds are quite mature. Lots of prom dresses around in shops at the moment which might provide options. Quiz has a lot of long dresses.