Evening/smart dress for 12 year old DD

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DD has a formal xmas party to attend.
She is 12 but very petite so can't get away with a size six.
She is terrified to looking like a little girl.

Any suggestions?

Bhs or new look do lovely prom dresses

Asos have a selection of dresses in size 4

this is pretty

some nice miss selfridge bits in size 4 too

Thank you everyone.
I'm going to send off for a couple and see how the sizing comes up.