Congratulations, Wendy Bird - and Peter Pan! That's great news.
I don't know whether Wendy's costume still exists somewhere but in the Peter Pan Keepsake published in 1907, the story of the play retold by Daniel O'Connor, with photos from the first stage production, there is a picture of Wendy in that costume in the Underground Home scene with this description in the text:
"Wendy looked very charming in her pretty brown frock, the colour of autumn leaves, with scarlet berries in her hair.."
In the beginning of the play/story, she is wearing a full long white nightgown.
Peter's costume, as played by Nina Boucicault was a red tunic worn over a white shirt with leaves sewn onto it. (See my post on 'Peter Pan in Red' strand).