Seven designed the holodeck program. It shows the way she thinks about herself & her relationships with others.
Most clearly, it shows that she thinks of herself as not fully human and not deserving of the things she desires (quarters, privacy, companionship) until she is "fully human." The tragic thing about that is that she never will be.
This is hardly a child's development.
7 is never going to be easily labeled with an age other than physical, and even then she's Borg enhanced. She has had a very complicated existence that neither she or anyone else on board completely understand. It's not like she was in stasis for all those years, she retains much of her Borg life.
I think she suffers more from never having been
a child of 7, 8, 9.. and a teenager than from purportedly being one because she was assimilated at 6.
Assimilated, grown in a vat, and then had her brain dumped with millions of individuals pains and experiences. She is not a child and she can never be a child again.