"She wants to have children."
Translation:Hun vil have børn.
Because using "gerne" is a way of politely asking for something, or politely asking if someone would like something. In this case, it's just stating she wants children.
If you said, "Hun vil gerne have børn", it would probably sound funny as if she was requesting to have "some children please" or something haha. This is just my humble opinion as a very non-fluent person though.