"Didn't want to do X until Y happened" vs. "hadn't wanted to do X until Y happened"

Which answer is correct?

Tony _______to have children until his little daughter was born. After she won his heart, he decided he wanted a big family.

A. didn't want.
B. hadn't wanted.

This is a multiple choice question from the Understanding and Using English Grammar Workbook 3rd edition by Betty Azar. To me, both answers are correct as I cannot see the diffrence. Please show me the wrong option to delete.

Everyone has explained very well why the past perfect is the correct answer. I'll just mention this. The reason that the simple past isn't correct is simply context. It doesn't make sense to say that you don't want to have a child until your child is born (it goes without saying, as we say), so it doesn't make sense to say that you didn't want to have children until your child was born, either. The second sentence makes it more clear that he changed his mind about having children once his first child was born.

Now, if the context were different, the simple past would be entirely correct: "I didn't want to have children until I found a job." Compare this with "I hadn't wanted to have children until I found a job, but then I won $100 million in a lottery." You can see (I hope) that context is what implies which tense to use.

He hadn't wanted children until then.

Saying 'didn't want ... until ...' would imply that he had always wanted the event to occur, but not before the condition was met. "I didn't want to come over until you were home."

In this case he did not want the event to occur, but then changed his mind. That establishes a point in the past when he changed his mind, and places the 'not wanting' before that point, calling for a past perfect. "I had'nt wanted to come over until I heard you all having such fun."

As the ongoing war against potentially superfluous commas advances, you might write option A in place of "He didn't want to have children, until his daughter was born." This says he had a confirmed state of mind, which is now over, for good. Then you would expect the followup sentence to be "Now that she has won his heart, he has decided he wants a big family" Since the past is spoken for, we would need to frame the followup with 'want' in the present tense, and push the other events around to make logical sense.

In this context didn't is correct because we are referring to something definite and in the past. If we were talking about something in past that was an alternative or unreal - did not actually happen, then we would use hadn't :

If I hadn't eaten so much, my stomach would not hurt.

EDIT:

I am reversing my opinion.... until establishes a past alternative, so Pick option B

http://www.englishpage.com/verbpage/pastperfect.html

Now, apply this to your sentence:

Tony had not wanted to have children until his little daughter was born.

Past Event 1: Tony had not wanted to have a child (this was a continuous event until...)

Past Event 2: Tony had a child. (Which discontinued Event 1.)

This is an appropriate use of the past-perfect tense.