The idea of moral ambiguity in Louisa May Alcott’s Little Women is a broad topic and covers a wide spectrum of ideas. I am guessing that you will be focusing on one of the two primary ideological ambivalences in this text. The lesser of the two ideas concerns the novel as a literary work as opposed to a conduct manual for “little women.” The more significant idea, of course, is the ambivalence between adhering to and independence from traditional female roles. This is the one I will be focusing on.
Since I do not know precisely what kind of essay you are writing, I can probably serve you best by offering some bits and pieces, ideas which may or may not quite fit your exact thesis but which should prompt you to apply what works to your essay.
Ambivalence seems to be the key to both a title and an effective introduction, so think about the idea of a tension between two things. I think of a short legend in which an old Native American sage is says that he has two dogs inside him, one which is evil and one which is good. These two dogs are in a constant fight with one another. When asked which dog wins, the wise old man pauses and says, "Whichever dog I feed." If you are writing a persuasive essay, this kind of thing would work well because you can address the idea that there are two competing philosophies presented in this novel and, presumably, you will make the case for which one you think is dominant, best, or more valid. You will tell us, in your writing, which ideology ("dog") you think Alcott feeds most.
Another way to look at that ambivalence or tension is to think about the idea of a split. There is a duality and a dichotomy to being a woman in the world of the March girls, and that has not changed so very much today in many ways. Society expects women to "do it all," so perhaps a reference to someone recognizable who manages to be both traditional and independent--or to someone who fails miserably at both. This might be especially effective if you plan to concentrate your writing on one of the girls.
Instead of concentrating on the ambivalence piece of your thesis, perhaps you will want to focus on the two ideologies which create that tension: traditional versus independent. Your essay will talk about how that looks for the March girls in the 1860s, so consider what does that look like today. What is traditional and what is independent today.
It is always acceptable to use an effective quote as long as it leads directly to your thesis, so perhaps choose a bit from the novel which you are not using in the essay but which makes your point. I think of Marmee's statement:
“If you feel your value lies in being merely decorative, I fear that someday you might find yourself believing that’s all that you really are. Time erodes all such beauty, but what it cannot diminish is the wonderful workings of your mind: Your humor, your kindness, and your moral courage. These are the things I cherish so in you. I so wish I could give my girls a more just world. But I know you’ll make it a better place."
This is an excellent jumping-off point for an essay on ideological ambiguity, but you could of course use a statement from outside of the novel, as well.
Again, tension, ambiguity, duality, and dichotomy all work as starting points for an intro. If your essay is a bit more caustic and will say that Alcott should have made her women stronger and less compliant with the traditional female role, words like betrayal and repression will work, as well.
Finally, a kind of dictionary definition of fulfillment and how the novel and the March girls might be effective for an introduction. Is it being more traditional or is it being more independent--and how is one to know?