Young Charlie possibly wanted a father figure
I know a lot of people have said that there was possible sexual tension between Young Charlie and Uncle Charlie. Knowing how "out there" Hitchcock was it would not put it past him to hint incest.
I also think that Charlie was also in need of a father figure. Even though Charlie has a father, it is evident in the film that the two don't have much of a relationship. The dad does not understand what his daughter wants and needs and does not bother to spend time with his kids. Even though Charlie is really smart, the father does not seem to be proud or enthiuastic the way most parents would. He comes home and does not have a real conversation with his family, he prefers talking about murder with his friend.
Let's also look at the physical constrast between her father and Charlie. Her dad is older,short, not handsome, looks out of shape and is stagnant. Charlie is the opposite where he is young, handsome, adventourous,and energetic.
I think that young Charlie saw her uncle as the opposite of what her family was and she yearned for his attention because she knew that it is what he can give to her.