Little Nell is one of the most criticized characters in Charles Dickens's oeuvre. She has been viewed as too sentimental, too passive, and subservient to the male characters of the novel. While some criticisms are more valid than others, to say Little Nell is only motivated by service to male authority figures misses one key element of her characterization: namely, the blend of both the child and the burgeoning adult woman within her.
Though Nell is a symbol of childhood innocence, she is also mature beyond her years in some ways. Examining Nell's dynamic with her grandfather, for instance, we see that she is not so much his servant as his caretaker: a young girl forced to treat a grown man like a little boy much of the time. The grandfather acts recklessly to the point where Nell carries all the money and tries to reason with him about how they should spend it. Nell is the more responsible of the two, but as she is only a child, she has to hand over the money if her grandfather absolutely insists upon it. However, he often relies upon her for comfort and encouragement as they make their journey through the countryside. In fact, when Nell dies, her grandfather is unable to function without her, much like how a child is lost without its mother.
If anything, Nell's death might be a criticism of how Nell has been forced to take on such a role even though she is still a young girl. As she dies, she thinks of all the people who helped her along her journey—the people who acted in service to her and not the other way around—and asks God to bless them. So, Robson's statement, while not entirely without merit, does not encompass the whole of Nell's character or her lamentable situation, which Dickens likely views as unnatural for a child.