As many of the commenters have said, there are black pants designed for women and sold in the women's section of the store. I don't think that qualifies as cross dressing. It's not like they were demanding that she wear men's pants.
The one pentecostal person who was speaking in the comments said that the creation of women's styles of pants are a relatively new thing (only around in the last century) and he feels that women should stick to more traditional female clothing. I can see his point in that. I think that there really has been a disturbing trend to try to make women more masculine and men more feminine and that this is something that needs to be countered by moral people. I don't personally think it's bad for women to wear pants designed for women though.
If this is about modesty, then pants are way more modest than skirts. If you don't believe me watch a playground of young kids and see the boys try to get the girls wearing skirts to "do a cartwheel". If a girl wearing girl's slacks does a cartwheel there's nothing showing but if she's wearing a skirt! Well... not so good outcome there.
With all that said though I think that as long as there are no safety issues that the girl should be allowed to wear a skirt, and the church has the right to hold to a more strict/traditional viewpoint. It's better in many ways than being too loose with the rules although I also think too much legalism can also be stifling.