In addition, Nora is a child because she isn't worldly and doesn't know who she really is apart from her husband and children. Her identity is completely contingent on being a wife and mother, and even these relationships are either extremely superficial and/or based on deceit. Ann Marie, the nurse, is the true mother, while Nora plays hide-and-seek with the kids, as if she were their playdate. Nora discusses "play" money and clothes with her husband in a silly, flirtatious matter while her husband calls her several bird "pet names". She never manages to get to the soul of the relationship until she decides to leave it.
She also doesn't realize the consequences behind illegal activity such as forgery. She feels that Krogstad would never expose her because she did it only with helping her husband in mind. If she were more mature and wordly, she would understand that the law is the law.
Although Nora is physically a woman, there is some credence in calling her a child, for she is completely dependant on her husband. She has a child-like energy too, as she is often "flighty and excitable." She also lacks the maturity to face situations and prefers to lie to cover things up when she makes errors. She is bad with money, seemingly not able to manage the funds for which she is responsible. For all of these reasons, it is easy to see why Mr. Linde describes Nora as a child.