The original outfits for boys and girls look the same but they have different game meshes. So I was able to replace both with another mesh and adjusted the gender so they would be unisex.
I don't have pictures of the following defaults but I think you all know what they look like. I replaced the soccer outfits for children whit some of Fakepeeps' clothing. Girls get these ones and boys will train in these ones. I made the outfits athletic only so they won't appear in the everyday section.