Symbolism, Imagery, Allegory
Here's another image from James's preface that does important work when it comes to setting the tone for the book as a whole:
What was clear to any spectator was that the only link binding [Maisie] to either parent was this lamentable fact of her being a ready vessel for bitterness, a deep little porcelain cup in which biting acids could be mixed. (Preface.6)
Sounds harsh, we know. But that's James's whole point: even at age six, Maisie will have to deal with everyone else's awful negativity. She'll somehow have to convert "biting acids" into nourishment, and the marvelous thing about her is that, with Mrs. Wix's help, she finally finds a way.
But let's break this quote down. James continues to describe Maisie in vessel, or cup-like, terms throughout the novel. By referring to her in this way, James is saying that innocence is a state of lack. When you're innocent, you're like an empty cup, waiting to be filled up with orange juice or tea or vinegar or, yeesh, biting acids. Fair enough, right? Today, we use expressions like "blank slate" or "empty canvas" when we talk about kiddos and innocence in general.
This wasn't exactly the case back in the Victorian era, though. Children were thought to already exist in a state of innocence, and experience— especially sexual experience, and especially for girls—was thought to "tarnish" that. Innocence wasn't a cup; it was more like a white cloth that life covered in the ketchup stains and coffee rings and muddy footprints of experience.
See the difference? James is firmly in the "nurture" camp of childhood development (fill that vessel with something good!), while social thought of the time was pretty much Team Nature (don't soil that already perfect kid!). James, with his handy-dandy vessel metaphor, shows that experience a) is not always bad and b) needs to be closely monitored by parents.
We discuss in "What's Up With the Title?" and elsewhere that What Maisie Knew was pretty radical at the time. This symbol of the vessel gets right to the heart of that. James isn't portraying a little girl as necessarily good and pure—she isn't a white cloth. Instead, she is just a small version of a human: she has foibles and wants and confusion just like the rest of us. However, because she is a child, she can be filled with positive, enriching experiences and knowledge (like a nourishing soup) instead of damaging, negative, soul-crushing ones (like acid). Us adults are already filled vessels—and to be honest, we're probably filled with the emotional equivalent of what happens when you mix all of the sodas at the soda fountain into one Big Gulp container.