What are Babich's particular ideas, forces, or trends behind the lack of female protagonists in kids' movie.

I think the reason is simply sexism on the part of studios as well as the audience who consumes these films:

Though we’d all like to think we’ve taken strides forward in terms of sexism in children’s movies, it seems filmmakers are having difficulty separating the concept of “heroine” from the concept of “sexiness.” But if most kids’ movies are going to be about animals and objects, rather than human beings, that’s what needs to happen if the female protagonist is to finally be revived.