In "El Dorado", Joey McDonald, the daughter of Kevin McDonald, is wearing pants whenever she rides a horse. Was it socially acceptable for a girl to wear pants in the Wild West?
Absolutely. There's very little writing on the subject, which in itself could be considered evidence that it wasn't so outrageous, but we do have a lot of photographs of frontierswomen wearing trousers, most of which I can find are from the book Women in Pants: Manly Maidens, Cowgirls, and Other Renegades:
There are also images of unknown women in trousers from the era, for example the name of this 1880s Arizona Scout has been lost, but it does suggest that men and women both wore the same type of clothing, as clearly a dress is not appropriate for battle or sleeping in the wilderness.