The woman is being sarcastic. Sarcasm is he use of remarks in way that says the opposite of what you mean in order to insult someone or show them that you are annoyed.
Normally, "little charmer" means means someone who is pleasant, causes you to feel happy. However, the child spilled a drink on the women. This does not make her happy. Therefore, she replied, "I take it this little charmer is yours" She is annoyed and uses sarcasm.