I want to eat = Jeg vil spise
I just wanted to see you = Jeg ville bare se dig
I have wanted to drive a Ferrari since I was a child = Jeg har villet køre en Ferrari siden jeg var barn.
Probably because A) this unit is about the present perfect, so they want only that tense and B) because the imperfect (at least in English) would indicate that she can no longer have children (her childbearing years are finished) or that she has changed her mind and now no longer wants children. Whereas, the present perfect, because of its connection to the present, indicates that she still can or still wants to have children today. Compare: "She always wanted children, until she met 'Johnny the Terrible Two-Year Old'" and now children are the last thing she wants!" vs "She has always wanted children and now that she is pregnant, she is very happy!" The two tenses aren't interchangeable, and from the 'feel' of the sentences in this unit (so far) the present perfect would be the only choice.