Does having a child has a negative/positive impact in karma? The question aims towards to creating a new life and not to the desire from which said life is concived. Other way of this question would be: the act of giving a life which can be seen as a being with it's own karma, creates more karma to one self?
 Has the buddha ever talked about this?
It depends on the intention behind the action. If the intention is unwholesome then it will lead to unwhole resultants and vice versa if its wholesome. It is the intention that has kammic potency.
Here is a quote from the wiki about what the Buddha said when he was about to leave his palace and at the same time got news about his newborn son:

&quot;Prince Siddhartha was preparing himself to leave the palace. One account claims that when he received the news of his son's birth he replied Rāhu jāto, bandhanam jātam — &quot;A rāhu is born, a fetter has arisen.&quot; Accordingly the child was named Rāhula, meaning &quot;fetter&quot;, or &quot;impediment&quot;, recognizing that the child could be a tie that bound him to his wife Yashodhara, a binding that may impede a search for enlightenment. Others, however, feel rāhu does not mean &quot;fetter&quot; in this sense. The second account, found in the Mūlasarvāstivāda vinaya, is that Rāhula received his name in accordance with an eclipse of the moon, caused by the snake Rahu. In Japanese he is called Ragora (羅睺羅).
Supporting the first account, in the Dhammapada, the pleasure and joy that a man receives in his wife and children is called a &quot;soft fetter&quot; that ties individuals to life and suffering, not just through eventual loss and separation of loved ones but more deeply and subtly may act as ties to cyclic existence (samsara)&quot;.